\chapter{Supervisor-Level ISA, Version 1.12} \label{supervisor} This chapter describes the RISC-V supervisor-level architecture, which contains a common core that is used with various supervisor-level address translation and protection schemes. \begin{commentary} Supervisor mode is deliberately restricted in terms of interactions with underlying physical hardware, such as physical memory and device interrupts, to support clean virtualization. In this spirit, certain supervisor-level facilities, including requests for timer and interprocessor interrupts, are provided by implementation-specific mechanisms. In some systems, a supervisor execution environment (SEE) provides these facilities in a manner specified by a supervisor binary interface (SBI). Other systems supply these facilities directly, through some other implementation-defined mechanism. \end{commentary} \section{Supervisor CSRs} A number of CSRs are provided for the supervisor. \begin{commentary} The supervisor should only view CSR state that should be visible to a supervisor-level operating system. In particular, there is no information about the existence (or non-existence) of higher privilege levels (machine level or other) visible in the CSRs accessible by the supervisor. Many supervisor CSRs are a subset of the equivalent machine-mode CSR, and the machine-mode chapter should be read first to help understand the supervisor-level CSR descriptions. \end{commentary} \subsection{Supervisor Status Register (\tt sstatus)} \label{sstatus} The {\tt sstatus} register is an SXLEN-bit read/write register formatted as shown in Figure~\ref{sstatusreg-rv32} for RV32 and Figure~\ref{sstatusreg} for RV64. The {\tt sstatus} register keeps track of the processor's current operating state. \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \scalebox{0.95}{ \begin{tabular}{cWcccccWccccWcc} \\ \instbit{31} & \instbitrange{30}{20} & \instbit{19} & \instbit{18} & \instbit{17} & \instbitrange{16}{15} & \instbitrange{14}{13} & \instbitrange{12}{9} & \instbit{8} & \instbit{7} & \instbit{6} & \instbit{5} & \instbitrange{4}{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{SD} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{MXR} & \multicolumn{1}{c|}{SUM} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{XS[1:0]} & \multicolumn{1}{c|}{FS[1:0]} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{SPP} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{UBE} & \multicolumn{1}{c|}{SPIE} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{SIE} & \multicolumn{1}{c|}{\wpri} \\ \hline 1 & 11 & 1 & 1 & 1 & 2 & 2 & 4 & 1 & 1 & 1 & 1 & 3 & 1 & 1 \\ \end{tabular}} \end{center} } \vspace{-0.1in} \caption{Supervisor-mode status register ({\tt sstatus}) for RV32.} \label{sstatusreg-rv32} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{cMFScccc} \\ \instbit{63} & \instbitrange{62}{34} & \instbitrange{33}{32} & \instbitrange{31}{20} & \instbit{19} & \instbit{18} & \instbit{17} & \\ \hline \multicolumn{1}{|c|}{SD} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{UXL[1:0]} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{MXR} & \multicolumn{1}{c|}{SUM} & \multicolumn{1}{c|}{\wpri} & \\ \hline 1 & 29 & 2 & 12 & 1 & 1 & 1 & \\ \end{tabular} \begin{tabular}{cWWFccccWcc} \\ & \instbitrange{16}{15} & \instbitrange{14}{13} & \instbitrange{12}{9} & \instbit{8} & \instbit{7} & \instbit{6} & \instbit{5} & \instbitrange{4}{2} & \instbit{1} & \instbit{0} \\ \hline & \multicolumn{1}{|c|}{XS[1:0]} & \multicolumn{1}{c|}{FS[1:0]} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{SPP} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{UBE} & \multicolumn{1}{c|}{SPIE} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{SIE} & \multicolumn{1}{c|}{\wpri} \\ \hline & 2 & 2 & 4 & 1 & 1 & 1 & 1 & 3 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Supervisor-mode status register ({\tt sstatus}) for RV64.} \label{sstatusreg} \end{figure*} The SPP bit indicates the privilege level at which a hart was executing before entering supervisor mode. When a trap is taken, SPP is set to 0 if the trap originated from user mode, or 1 otherwise. When an SRET instruction (see Section~\ref{otherpriv}) is executed to return from the trap handler, the privilege level is set to user mode if the SPP bit is 0, or supervisor mode if the SPP bit is 1; SPP is then set to 0. The SIE bit enables or disables all interrupts in supervisor mode. When SIE is clear, interrupts are not taken while in supervisor mode. When the hart is running in user-mode, the value in SIE is ignored, and supervisor-level interrupts are enabled. The supervisor can disable individual interrupt sources using the {\tt sie} CSR. The SPIE bit indicates whether supervisor interrupts were enabled prior to trapping into supervisor mode. When a trap is taken into supervisor mode, SPIE is set to SIE, and SIE is set to 0. When an SRET instruction is executed, SIE is set to SPIE, then SPIE is set to 1. The {\tt sstatus} register is a subset of the {\tt mstatus} register. \begin{commentary} In a straightforward implementation, reading or writing any field in {\tt sstatus} is equivalent to reading or writing the homonymous field in {\tt mstatus}. \end{commentary} \subsubsection{Base ISA Control in {\tt sstatus} Register} The UXL field controls the value of XLEN for U-mode, termed {\em UXLEN}, which may differ from the value of XLEN for S-mode, termed {\em SXLEN}. The encoding of UXL is the same as that of the MXL field of {\tt misa}, shown in Table~\ref{misabase}. For RV32 systems, the UXL field does not exist, and UXLEN=32. For RV64 systems, it is a \warl\ field that encodes the current value of UXLEN. In particular, an implementation may make UXL be a read-only field whose value always ensures that UXLEN=SXLEN. If UXLEN~$\ne$~SXLEN, instructions executed in the narrower mode must ignore source register operand bits above the configured XLEN, and must sign-extend results to fill the widest supported XLEN in the destination register. If UXLEN~$<$~SXLEN, user-mode instruction-fetch addresses and load and store effective addresses are taken modulo $2^{\text{UXLEN}}$. For example, when UXLEN=32 and SXLEN=64, user-mode memory accesses reference the lowest \wunits{4}{GiB} of the address space. \subsubsection{Memory Privilege in {\tt sstatus} Register} \label{sec:sum} The MXR (Make eXecutable Readable) bit modifies the privilege with which loads access virtual memory. When MXR=0, only loads from pages marked readable (R=1 in Figure~\ref{sv32pte}) will succeed. When MXR=1, loads from pages marked either readable or executable (R=1 or X=1) will succeed. MXR has no effect when page-based virtual memory is not in effect. The SUM (permit Supervisor User Memory access) bit modifies the privilege with which S-mode loads and stores access virtual memory. When SUM=0, S-mode memory accesses to pages that are accessible by U-mode (U=1 in Figure~\ref{sv32pte}) will fault. When SUM=1, these accesses are permitted. SUM has no effect when page-based virtual memory is not in effect, nor when executing in U-mode. Note that S-mode can never execute instructions from user pages, regardless of the state of SUM. SUM is hardwired to 0 if {\tt satp}.MODE is hardwired to 0. \begin{commentary} The SUM mechanism prevents supervisor software from inadvertently accessing user memory. Operating systems can execute the majority of code with SUM clear; the few code segments that should access user memory can temporarily set SUM. The SUM mechanism does not avail S-mode software of permission to execute instructions in user code pages. Legitimate uses cases for execution from user memory in supervisor context are rare in general and nonexistent in POSIX environments. However, bugs in supervisors that lead to arbitrary code execution are much easier to exploit if the supervisor exploit code can be stored in a user buffer at a virtual address chosen by an attacker. Some non-POSIX single address space operating systems do allow certain privileged software to partially execute in supervisor mode, while most programs run in user mode, all in a shared address space. This use case can be realized by mapping the physical code pages at multiple virtual addresses with different permissions, possibly with the assistance of the instruction page-fault handler to direct supervisor software to use the alternate mapping. \end{commentary} \subsubsection{Endianness Control in {\tt sstatus} Register} The UBE bit is a \warl\ field that controls the endianness of explicit memory accesses made from U-mode, which may differ from the endianness of memory accesses in S-mode. An implementation may make UBE be a read-only field that always specifies the same endianness as for S-mode. UBE controls whether explicit load and store memory accesses made from U-mode are little-endian (UBE=0) or big-endian (UBE=1). UBE has no effect on instruction fetches, which are {\em implicit} memory accesses that are always little-endian. For {\em implicit} accesses to supervisor-level memory management data structures, such as page tables, S-mode endianness always applies and UBE is ignored. \begin{commentary} Standard RISC-V ABIs are expected to be purely little-endian-only or big-endian-only, with no accommodation for mixing endianness. Nevertheless, endianness control has been defined so as to permit an OS of one endianness to execute user-mode programs of the opposite endianness. \end{commentary} \subsection{Supervisor Trap Vector Base Address Register ({\tt stvec})} The {\tt stvec} register is an SXLEN-bit read/write register that holds trap vector configuration, consisting of a vector base address (BASE) and a vector mode (MODE). \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{J@{}R} \instbitrange{SXLEN-1}{2} & \instbitrange{1}{0} \\ \hline \multicolumn{1}{|c|}{BASE[SXLEN-1:2] (\warl)} & \multicolumn{1}{c|}{MODE (\warl)} \\ \hline SXLEN-2 & 2 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Supervisor trap vector base address register ({\tt stvec}).} \label{stvecreg} \end{figure*} The BASE field in {\tt stvec} is a \warl\ field that can hold any valid virtual or physical address, subject to the following alignment constraints: the address must be 4-byte aligned, and MODE settings other than Direct might impose additional alignment constraints on the value in the BASE field. \begin{table*}[h!] \begin{center} \begin{tabular}{|r|c|l|} \hline Value & Name & Description \\ \hline 0 & Direct & All exceptions set {\tt pc} to BASE. \\ 1 & Vectored & Asynchronous interrupts set {\tt pc} to BASE+4$\times$cause. \\ $\ge$2 & --- & {\em Reserved} \\ \hline \end{tabular} \end{center} \caption{Encoding of {\tt stvec} MODE field.} \label{stvec-mode} \end{table*} The encoding of the MODE field is shown in Table~\ref{stvec-mode}. When MODE=Direct, all traps into supervisor mode cause the {\tt pc} to be set to the address in the BASE field. When MODE=Vectored, all synchronous exceptions into supervisor mode cause the {\tt pc} to be set to the address in the BASE field, whereas interrupts cause the {\tt pc} to be set to the address in the BASE field plus four times the interrupt cause number. For example, a supervisor-mode timer interrupt (see Table~\ref{scauses}) causes the {\tt pc} to be set to BASE+{\tt 0x14}. Setting MODE=Vectored may impose a stricter alignment constraint on BASE. \subsection{Supervisor Interrupt Registers ({\tt sip} and {\tt sie})} The {\tt sip} register is an SXLEN-bit read/write register containing information on pending interrupts, while {\tt sie} is the corresponding SXLEN-bit read/write register containing interrupt enable bits. Interrupt cause number \textit{i} (as reported in CSR {\tt scause}, Section~\ref{sec:scause}) corresponds with bit~\textit{i} in both {\tt sip} and {\tt sie}. Bits 15:0 are allocated to standard interrupt causes only, while bits 16 and above are designated for platform or custom use. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{SXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{Interrupts (\warl)} \\ \hline SXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Supervisor interrupt-pending register ({\tt sip}).} \label{sipreg} \end{figure} \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{SXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{Interrupts (\warl)} \\ \hline SXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Supervisor interrupt-enable register ({\tt sie}).} \label{siereg} \end{figure} An interrupt \textit{i} will be taken if bit \textit{i} is set in both {\tt sip} and {\tt sie}, and if supervisor-level interrupts are globally enabled. Supervisor-level interrupts are globally enabled if the hart's current privilege mode is less than S, or if the current privilege mode is S and the SIE bit in the {\tt sstatus} register is set. Each individual bit in register {\tt sip} may be writable or may be read-only. When bit~\textit{i} in {\tt sip} is writable, a pending interrupt \textit{i} can be cleared by writing 0 to this bit. If interrupt \textit{i} can become pending but bit~\textit{i} in {\tt sip} is read-only, the implementation must provide some other mechanism for clearing the pending interrupt (which may involve a call to the execution environment). A bit in {\tt sie} must be writable if the corresponding interrupt can ever become pending. Bits of {\tt sie} that are not writable must be hardwired to zero. The standard portions (bits 15:0) of registers {\tt sip} and {\tt sie} are formatted as shown in Figures \ref{sipreg-standard} and \ref{siereg-standard} respectively. \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{ScFcFcc} \instbitrange{15}{10} & \instbit{9} & \instbitrange{8}{6} & \instbit{5} & \instbitrange{4}{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{0} & \multicolumn{1}{c|}{SEIP} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{STIP} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{SSIP} & \multicolumn{1}{c|}{0} \\ \hline 6 & 1 & 3 & 1 & 3 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Standard portion (bits 15:0) of {\tt sip}.} \label{sipreg-standard} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{ScFcFcc} \instbitrange{15}{10} & \instbit{9} & \instbitrange{8}{6} & \instbit{5} & \instbitrange{4}{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{0} & \multicolumn{1}{c|}{SEIE} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{STIE} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{SSIE} & \multicolumn{1}{c|}{0} \\ \hline 6 & 1 & 3 & 1 & 3 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Standard portion (bits 15:0) of {\tt sie}.} \label{siereg-standard} \end{figure*} Bits {\tt sip}.SEIP and {\tt sie}.SEIE are the interrupt-pending and interrupt-enable bits for supervisor-level external interrupts. If implemented, SEIP is read-only in {\tt sip}, and is set and cleared by the execution environment, typically through a platform-specific interrupt controller. Bits {\tt sip}.STIP and {\tt sie}.STIE are the interrupt-pending and interrupt-enable bits for supervisor-level timer interrupts. If implemented, STIP is read-only in {\tt sip}, and is set and cleared by the execution environment. Bits {\tt sip}.SSIP and {\tt sie}.SSIE are the interrupt-pending and interrupt-enable bits for supervisor-level software interrupts. If implemented, SSIP is writable in {\tt sip}. A supervisor-level software interrupt is triggered on the current hart by writing 1 to SSIP, while a pending supervisor-level software interrupt can be cleared by writing 0 to SSIP. \begin{commentary} Interprocessor interrupts are sent to other harts by implementation-specific means, which will ultimately cause the SSIP bit to be set in the recipient hart's {\tt sip} register. \end{commentary} Each standard interrupt type (SEI, STI, or SSI) may not be implemented, in which case the corresponding interrupt-pending and interrupt-enable bits are hardwired to zeros. All bits in {\tt sip} and {\tt sie} are \warl\ fields. The implemented interrupts may be found by writing one to every bit location in {\tt sie}, then reading back to see which bit positions hold a one. \begin{commentary} The {\tt sip} and {\tt sie} registers are subsets of the {\tt mip} and {\tt mie} registers. Reading any implemented field, or writing any writable field, of {\tt sip}/{\tt sie} effects a read or write of the homonymous field of {\tt mip}/{\tt mie}. Bits 3, 7, and 11 of {\tt sip} and {\tt sie} correspond to the machine-mode software, timer, and external interrupts, respectively. Since most platforms will choose not to make these interrupts delegatable from M-mode to S-mode, they are shown as hardwired to 0 in Figures~\ref{sipreg-standard} and \ref{siereg-standard}. \end{commentary} Multiple simultaneous interrupts destined for supervisor mode are handled in the following decreasing priority order: SEI, SSI, STI. Synchronous exceptions are of lower priority than all interrupts. \subsection{Supervisor Timers and Performance Counters} Supervisor software uses the same hardware performance monitoring facility as user-mode software, including the {\tt time}, {\tt cycle}, and {\tt instret} CSRs. The implementation should provide a mechanism to modify the counter values. The implementation must provide a facility for scheduling timer interrupts in terms of the real-time counter, {\tt time}. \subsection{Counter-Enable Register ({\tt scounteren})} \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{cccMcccccc} \instbit{31} & \instbit{30} & \instbit{29} & \instbitrange{28}{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{HPM31} & \multicolumn{1}{c|}{HPM30} & \multicolumn{1}{c|}{HPM29} & \multicolumn{1}{c|}{...} & \multicolumn{1}{c|}{HPM5} & \multicolumn{1}{c|}{HPM4} & \multicolumn{1}{c|}{HPM3} & \multicolumn{1}{c|}{IR} & \multicolumn{1}{c|}{TM} & \multicolumn{1}{c|}{CY} \\ \hline 1 & 1 & 1 & 23 & 1 & 1 & 1 & 1 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Counter-enable register ({\tt scounteren}).} \label{scounteren} \end{figure*} The counter-enable register {\tt scounteren} is a 32-bit register that controls the availability of the hardware performance monitoring counters to U-mode. When the CY, TM, IR, or HPM{\em n} bit in the {\tt scounteren} register is clear, attempts to read the {\tt cycle}, {\tt time}, {\tt instret}, or {\tt hpmcounter{\em n}} register while executing in U-mode will cause an illegal instruction exception. When one of these bits is set, access to the corresponding register is permitted. {\tt scounteren} must be implemented. However, any of the bits may contain a hardwired value of zero, indicating reads to the corresponding counter will cause an exception when executing in U-mode. Hence, they are effectively \warl\ fields. \begin{commentary} The setting of a bit in {\tt mcounteren} does not affect whether the corresponding bit in {\tt scounteren} is writable. However, U-mode may only access a counter if the corresponding bits in {\tt scounteren} and {\tt mcounteren} are both set. \end{commentary} \subsection{Supervisor Scratch Register ({\tt sscratch})} The {\tt sscratch} register is an SXLEN-bit read/write register, dedicated for use by the supervisor. Typically, {\tt sscratch} is used to hold a pointer to the hart-local supervisor context while the hart is executing user code. At the beginning of a trap handler, {\tt sscratch} is swapped with a user register to provide an initial working register. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{SXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{\tt sscratch} \\ \hline SXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Supervisor Scratch Register.} \label{kregs} \end{figure} \subsection{Supervisor Exception Program Counter ({\tt sepc})} {\tt sepc} is an SXLEN-bit read/write register formatted as shown in Figure~\ref{epcreg}. The low bit of {\tt sepc} ({\tt sepc[0]}) is always zero. On implementations that support only IALIGN=32, the two low bits ({\tt sepc[1:0]}) are always zero. If an implementation allows IALIGN to be either 16 or 32 (by changing CSR {\tt misa}, for example), then, whenever IALIGN=32, bit {\tt sepc[1]} is masked on reads so that it appears to be 0. This masking occurs also for the implicit read by the SRET instruction. Though masked, {\tt sepc[1]} remains writable when IALIGN=32. {\tt sepc} is a \warl\ register that must be able to hold all valid virtual addresses. It need not be capable of holding all possible invalid addresses. Implementations may convert some invalid address patterns into other invalid addresses prior to writing them to {\tt sepc}. When a trap is taken into S-mode, {\tt sepc} is written with the virtual address of the instruction that was interrupted or that encountered the exception. Otherwise, {\tt sepc} is never written by the implementation, though it may be explicitly written by software. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{SXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{\tt sepc} \\ \hline SXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Supervisor exception program counter register.} \label{epcreg} \end{figure} \subsection{Supervisor Cause Register ({\tt scause})} \label{sec:scause} The {\tt scause} register is an SXLEN-bit read-write register formatted as shown in Figure~\ref{scausereg}. When a trap is taken into S-mode, {\tt scause} is written with a code indicating the event that caused the trap. Otherwise, {\tt scause} is never written by the implementation, though it may be explicitly written by software. The Interrupt bit in the {\tt scause} register is set if the trap was caused by an interrupt. The Exception Code field contains a code identifying the last exception or interrupt. Table~\ref{scauses} lists the possible exception codes for the current supervisor ISAs. The Exception Code is a \wlrl\ field. It is required to hold the values 0--31 (i.e., bits 4--0 must be implemented), but otherwise it is only guaranteed to hold supported exception codes. \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{c@{}U} \instbit{SXLEN-1} & \instbitrange{SXLEN-2}{0} \\ \hline \multicolumn{1}{|c|}{Interrupt} & \multicolumn{1}{c|}{Exception Code (\wlrl)} \\ \hline 1 & SXLEN-1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Supervisor Cause register {\tt scause}.} \label{scausereg} \end{figure*} \begin{table*}[h!] \begin{center} \begin{tabular}{|r|r|l|l|} \hline Interrupt & Exception Code & Description \\ \hline 1 & 0 & {\em Reserved} \\ 1 & 1 & Supervisor software interrupt \\ 1 & 2--4 & {\em Reserved} \\ 1 & 5 & Supervisor timer interrupt \\ 1 & 6--8 & {\em Reserved} \\ 1 & 9 & Supervisor external interrupt \\ 1 & 10--15 & {\em Reserved} \\ 1 & $\ge$16 & {\em Designated for platform use} \\ \hline 0 & 0 & Instruction address misaligned \\ 0 & 1 & Instruction access fault \\ 0 & 2 & Illegal instruction \\ 0 & 3 & Breakpoint \\ 0 & 4 & Load address misaligned \\ 0 & 5 & Load access fault \\ 0 & 6 & Store/AMO address misaligned \\ 0 & 7 & Store/AMO access fault \\ 0 & 8 & Environment call from U-mode \\ 0 & 9 & Environment call from S-mode \\ 0 & 10--11 & {\em Reserved} \\ 0 & 12 & Instruction page fault \\ 0 & 13 & Load page fault \\ 0 & 14 & {\em Reserved} \\ 0 & 15 & Store/AMO page fault \\ 0 & 16--23 & {\em Reserved} \\ 0 & 24--31 & {\em Designated for custom use} \\ 0 & 32--47 & {\em Reserved} \\ 0 & 48--63 & {\em Designated for custom use} \\ 0 & $\ge$64 & {\em Reserved} \\ \hline \end{tabular} \end{center} \caption{Supervisor cause register ({\tt scause}) values after trap. Synchronous exception priorities are given by Table~\ref{exception-priority}.} \label{scauses} \end{table*} \subsection{Supervisor Trap Value ({\tt stval}) Register} The {\tt stval} register is an SXLEN-bit read-write register formatted as shown in Figure~\ref{stvalreg}. When a trap is taken into S-mode, {\tt stval} is written with exception-specific information to assist software in handling the trap. Otherwise, {\tt stval} is never written by the implementation, though it may be explicitly written by software. The hardware platform will specify which exceptions must set {\tt stval} informatively and which may unconditionally set it to zero. When a breakpoint, address-misaligned, access-fault, or page-fault exception occurs on an instruction fetch, load, or store, {\tt stval} is written with the faulting virtual address. On an illegal instruction trap, {\tt stval} may be written with the first XLEN or ILEN bits of the faulting instruction as described below. For other exceptions, {\tt stval} is set to zero, but a future standard may redefine {\tt stval}'s setting for other exceptions. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{SXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{\tt stval} \\ \hline SXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Supervisor Trap Value register.} \label{stvalreg} \end{figure} For misaligned loads and stores that cause access-fault or page-fault exceptions, {\tt stval} will contain the virtual address of the portion of the access that caused the fault. For instruction access-fault or page-fault exceptions on systems with variable-length instructions, {\tt stval} will contain the virtual address of the portion of the instruction that caused the fault while {\tt sepc} will point to the beginning of the instruction. The {\tt stval} register can optionally also be used to return the faulting instruction bits on an illegal instruction exception ({\tt sepc} points to the faulting instruction in memory). If this feature is not provided, then {\tt stval} is set to zero on an illegal instruction fault. If this feature is provided, after an illegal instruction trap, {\tt stval} will contain the shortest of: \begin{compactitem} \item the actual faulting instruction \item the first ILEN bits of the faulting instruction \item the first XLEN bits of the faulting instruction \end{compactitem} The value loaded into {\tt stval} is right-justified and all unused upper bits are cleared to zero. {\tt stval} is a \warl\ register that must be able to hold all valid virtual addresses and the value 0. It need not be capable of holding all possible invalid addresses. Implementations may convert some invalid address patterns into other invalid addresses prior to writing them to {\tt stval}. If the feature to return the faulting instruction bits is implemented, {\tt stval} must also be able to hold all values less than $2^N$, where $N$ is the smaller of XLEN and ILEN. \subsection{Supervisor Address Translation and Protection ({\tt satp}) Register} \label{sec:satp} The {\tt satp} register is an SXLEN-bit read/write register, formatted as shown in Figure~\ref{rv32satp} for SXLEN=32 and Figure~\ref{rv64satp} for SXLEN=64, which controls supervisor-mode address translation and protection. This register holds the physical page number (PPN) of the root page table, i.e., its supervisor physical address divided by \wunits{4}{KiB}; an address space identifier (ASID), which facilitates address-translation fences on a per-address-space basis; and the MODE field, which selects the current address-translation scheme. Further details on the access to this register are described in Section~\ref{virt-control}. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{c@{}E@{}K} \instbit{31} & \instbitrange{30}{22} & \instbitrange{21}{0} \\ \hline \multicolumn{1}{|c|}{{\tt MODE} (\warl)} & \multicolumn{1}{|c|}{{\tt ASID} (\warl)} & \multicolumn{1}{|c|}{{\tt PPN} (\warl)} \\ \hline 1 & 9 & 22 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{RV32 Supervisor address translation and protection register {\tt satp}.} \label{rv32satp} \end{figure} \begin{commentary} Storing a PPN in {\tt satp}, rather than a physical address, supports a physical address space larger than \wunits{4}{GiB} for RV32. The {\tt satp}.PPN field might not be capable of holding all physical page numbers. Some platform standards might place constraints on the values {\tt satp}.PPN may assume, e.g., by requiring that all physical page numbers corresponding to main memory be representable. \end{commentary} \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}S@{}T@{}U} \instbitrange{63}{60} & \instbitrange{59}{44} & \instbitrange{43}{0} \\ \hline \multicolumn{1}{|c|}{{\tt MODE} (\warl)} & \multicolumn{1}{|c|}{{\tt ASID} (\warl)} & \multicolumn{1}{|c|}{{\tt PPN} (\warl)} \\ \hline 4 & 16 & 44 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{RV64 Supervisor address translation and protection register {\tt satp}, for MODE values Bare, Sv39, and Sv48.} \label{rv64satp} \end{figure} \begin{commentary} We store the ASID and the page table base address in the same CSR to allow the pair to be changed atomically on a context switch. Swapping them non-atomically could pollute the old virtual address space with new translations, or vice-versa. This approach also slightly reduces the cost of a context switch. \end{commentary} Table~\ref{tab:satp-mode} shows the encodings of the MODE field for RV32 and RV64. When MODE=Bare, supervisor virtual addresses are equal to supervisor physical addresses, and there is no additional memory protection beyond the physical memory protection scheme described in Section~\ref{sec:pmp}. To select MODE=Bare, software must write zero to the remaining fields of {\tt satp} (bits 30--0 for RV32, or bits 59--0 for RV64). Attempting to select MODE=Bare with a nonzero pattern in the remaining fields has an \unspecified\ effect on the value that the remaining fields assume and an \unspecified\ effect on address translation and protection behavior. For RV32, the {\tt satp} encodings corresponding to MODE=Bare and ASID[8:7]=3 are designated for custom use, whereas the encodings corresponding to MODE=Bare and ASID[8:7]$\ne$3 are reserved for future standard use. For RV64, all {\tt satp} encodings corresponding to MODE=Bare are reserved for future standard use. \begin{commentary} Version 1.11 of this standard stated that the remaining fields in {\tt satp} had no effect when MODE=Bare. Making these fields reserved facilitates future definition of additional translation and protection modes, particularly in RV32, for which all patterns of the existing MODE field have already been allocated. \end{commentary} For RV32, the only other valid setting for MODE is Sv32, a paged virtual-memory scheme described in Section~\ref{sec:sv32}. For RV64, two paged virtual-memory schemes are defined: Sv39 and Sv48, described in Sections~\ref{sec:sv39} and \ref{sec:sv48}, respectively. Two additional schemes, Sv57 and Sv64, will be defined in a later version of this specification. The remaining MODE settings are reserved for future use and may define different interpretations of the other fields in {\tt satp}. Implementations are not required to support all MODE settings, and if {\tt satp} is written with an unsupported MODE, the entire write has no effect; no fields in {\tt satp} are modified. \begin{table}[h] \begin{center} \begin{tabular}{|c|c|l|} \hline \multicolumn{3}{|c|}{RV32} \\ \hline Value & Name & Description \\ \hline 0 & Bare & No translation or protection. \\ 1 & Sv32 & Page-based 32-bit virtual addressing (see Section~\ref{sec:sv32}). \\ \hline \hline \multicolumn{3}{|c|}{RV64} \\ \hline Value & Name & Description \\ \hline 0 & Bare & No translation or protection. \\ 1--7 & --- & {\em Reserved for standard use} \\ 8 & Sv39 & Page-based 39-bit virtual addressing (see Section~\ref{sec:sv39}). \\ 9 & Sv48 & Page-based 48-bit virtual addressing (see Section~\ref{sec:sv48}). \\ 10 & {\em Sv57} & {\em Reserved for page-based 57-bit virtual addressing.} \\ 11 & {\em Sv64} & {\em Reserved for page-based 64-bit virtual addressing.} \\ 12--13 & --- & {\em Reserved for standard use} \\ 14--15 & --- & {\em Designated for custom use} \\ \hline \end{tabular} \end{center} \caption{Encoding of {\tt satp} MODE field.} \label{tab:satp-mode} \end{table} The number of ASID bits is \unspecified\ and may be zero. The number of implemented ASID bits, termed {\mbox {\em ASIDLEN}}, may be determined by writing one to every bit position in the ASID field, then reading back the value in {\tt satp} to see which bit positions in the ASID field hold a one. The least-significant bits of ASID are implemented first: that is, if ASIDLEN~$>$~0, ASID[ASIDLEN-1:0] is writable. The maximal value of ASIDLEN, termed ASIDMAX, is 9 for Sv32 or 16 for Sv39 and Sv48. \begin{commentary} For many applications, the choice of page size has a substantial performance impact. A large page size increases TLB reach and loosens the associativity constraints on virtually indexed, physically tagged caches. At the same time, large pages exacerbate internal fragmentation, wasting physical memory and possibly cache capacity. After much deliberation, we have settled on a conventional page size of 4 KiB for both RV32 and RV64. We expect this decision to ease the porting of low-level runtime software and device drivers. The TLB reach problem is ameliorated by transparent superpage support in modern operating systems~\cite{transparent-superpages}. Additionally, multi-level TLB hierarchies are quite inexpensive relative to the multi-level cache hierarchies whose address space they map. \end{commentary} Note that writing {\tt satp} does not imply any ordering constraints between page-table updates and subsequent address translations. If the new address space's page tables have been modified, or if an ASID is reused, it may be necessary to execute an SFENCE.VMA instruction (see Section~\ref{sec:sfence.vma}) after writing {\tt satp}. \begin{commentary} Not imposing upon implementations to flush address-translation caches upon {\tt satp} writes reduces the cost of context switches, provided a sufficiently large ASID space. \end{commentary} \section{Supervisor Instructions} In addition to the SRET instruction defined in Section~\ref{otherpriv}, one new supervisor-level instruction is provided. \subsection{Supervisor Memory-Management Fence Instruction} \label{sec:sfence.vma} \vspace{-0.2in} \begin{center} \begin{tabular}{O@{}R@{}R@{}F@{}R@{}S} \\ \instbitrange{31}{25} & \instbitrange{24}{20} & \instbitrange{19}{15} & \instbitrange{14}{12} & \instbitrange{11}{7} & \instbitrange{6}{0} \\ \hline \multicolumn{1}{|c|}{funct7} & \multicolumn{1}{c|}{rs2} & \multicolumn{1}{c|}{rs1} & \multicolumn{1}{c|}{funct3} & \multicolumn{1}{c|}{rd} & \multicolumn{1}{c|}{opcode} \\ \hline 7 & 5 & 5 & 3 & 5 & 7 \\ SFENCE.VMA & asid & vaddr & PRIV & 0 & SYSTEM \\ \end{tabular} \end{center} The supervisor memory-management fence instruction SFENCE.VMA is used to synchronize updates to in-memory memory-management data structures with current execution. Instruction execution causes implicit reads and writes to these data structures; however, these implicit references are ordinarily not ordered with respect to explicit loads and stores. Executing an SFENCE.VMA instruction guarantees that any previous stores already visible to the current RISC-V hart are ordered before all subsequent implicit references from that hart to the memory-management data structures. Further details on the behavior of this instruction are described in Section~\ref{virt-control} and Section~\ref{pmp-vmem}. \begin{commentary} The SFENCE.VMA is used to flush any local hardware caches related to address translation. It is specified as a fence rather than a TLB flush to provide cleaner semantics with respect to which instructions are affected by the flush operation and to support a wider variety of dynamic caching structures and memory-management schemes. SFENCE.VMA is also used by higher privilege levels to synchronize page table writes and the address translation hardware. \end{commentary} SFENCE.VMA orders only the local hart's implicit references to the memory-management data structures. \begin{commentary} Consequently, other harts must be notified separately when the memory-management data structures have been modified. One approach is to use 1) a local data fence to ensure local writes are visible globally, then 2) an interprocessor interrupt to the other thread, then 3) a local SFENCE.VMA in the interrupt handler of the remote thread, and finally 4) signal back to originating thread that operation is complete. This is, of course, the RISC-V analog to a TLB shootdown. \end{commentary} For the common case that the translation data structures have only been modified for a single address mapping (i.e., one page or superpage), {\em rs1} can specify a virtual address within that mapping to effect a translation fence for that mapping only. Furthermore, for the common case that the translation data structures have only been modified for a single address-space identifier, {\em rs2} can specify the address space. The behavior of SFENCE.VMA depends on {\em rs1} and {\em rs2} as follows: \begin{itemize} \item If {\em rs1}={\tt x0} and {\em rs2}={\tt x0}, the fence orders all reads and writes made to any level of the page tables, for all address spaces. \item If {\em rs1}={\tt x0} and {\em rs2}$\neq${\tt x0}, the fence orders all reads and writes made to any level of the page tables, but only for the address space identified by integer register {\em rs2}. Accesses to {\em global} mappings (see Section~\ref{sec:translation}) are not ordered. \item If {\em rs1}$\neq${\tt x0} and {\em rs2}={\tt x0}, the fence orders only reads and writes made to leaf page table entries corresponding to the virtual address in {\em rs1}, for all address spaces. \item If {\em rs1}$\neq${\tt x0} and {\em rs2}$\neq${\tt x0}, the fence orders only reads and writes made to leaf page table entries corresponding to the virtual address in {\em rs1}, for the address space identified by integer register {\em rs2}. Accesses to global mappings are not ordered. \end{itemize} When {\em rs2}$\neq${\tt x0}, bits SXLEN-1:ASIDMAX of the value held in {\em rs2} are reserved for future use and should be zeroed by software and ignored by current implementations. Furthermore, if ASIDLEN~$<$~ASIDMAX, the implementation shall ignore bits ASIDMAX-1:ASIDLEN of the value held in {\em rs2}. \begin{commentary} Simpler implementations can ignore the virtual address in {\em rs1} and the ASID value in {\em rs2} and always perform a global fence. \end{commentary} Implementations may perform implicit reads of the translation data structures pointed to by the current {\tt satp} register arbitrarily early and speculatively. The results of these reads may be held in an incoherent cache but not shared with other harts. Cache entries may only be established for the ASID currently loaded into the {\tt satp} register, or for global entries. The cache may only satisfy implicit reads for entries that have been established for the ASID currently loaded into {\tt satp}, or for global entries. Changes in the {\tt satp} register do not necessarily flush any such translation caches. To ensure the implicit reads observe writes to the same memory locations, an SFENCE.VMA instruction must be executed after the writes to flush the relevant cached translations. \begin{commentary} A consequence of this specification is that an implementation may use any translation for an address that was valid at any time since the most recent SFENCE.VMA that subsumes that address. In particular, if a leaf PTE is modified but a subsuming SFENCE.VMA is not executed, either the old translation or the new translation will be used, but the choice is unpredictable. The behavior is otherwise well-defined. In a conventional TLB design, it is possible for multiple entries to match a single address if, for example, a page is upgraded to a superpage without first clearing the original non-leaf PTE's valid bit and executing an SFENCE.VMA with {\em rs1}={\tt x0}. In this case, a similar remark applies: it is unpredictable whether the old non-leaf PTE or the new leaf PTE is used, but the behavior is otherwise well defined. \end{commentary} \begin{commentary} This specification permits the caching of PTEs whose V (Valid) bit is clear. Operating systems must be written to cope with this possibility, but implementers are reminded that eagerly caching invalid PTEs will reduce performance by causing additional page faults. \end{commentary} Implementations must only perform implicit reads of the translation data structures pointed to by the current contents of the {\tt satp} register or a subsequent valid (V=1) translation data structure entry, and must only raise exceptions for implicit accesses that are generated as a result of instruction execution, not those that are performed speculatively. Changes to the {\tt sstatus} fields SUM and MXR take effect immediately, without the need to execute an SFENCE.VMA instruction. Changing {\tt satp}.MODE from Bare to other modes and vice versa also takes effect immediately, without the need to execute an SFENCE.VMA instruction. Likewise, changes to {\tt satp}.ASID take effect immediately. \begin{commentary} The following common situations typically require executing an SFENCE.VMA instruction: \vspace{-0.1in} \begin{itemize} \item When software recycles an ASID (i.e., reassociates it with a different page table), it should {\em first} change {\tt satp} to point to the new page table using the recycled ASID, {\em then} execute SFENCE.VMA with {\em rs1}={\tt x0} and {\em rs2} set to the recycled ASID. Alternatively, software can execute the same SFENCE.VMA instruction while a different ASID is loaded into {\tt satp}, provided the next time {\tt satp} is loaded with the recycled ASID, it is simultaneously loaded with the new page table. \item If the implementation does not provide ASIDs, or software chooses to always use ASID 0, then after every {\tt satp} write, software should execute SFENCE.VMA with {\em rs1}={\tt x0}. In the common case that no global translations have been modified, {\em rs2} should be set to a register other than {\tt x0} but which contains the value zero, so that global translations are not flushed. \item If software modifies a non-leaf PTE, it should execute SFENCE.VMA with {\em rs1}={\tt x0}. If any PTE along the traversal path had its G bit set, {\em rs2} must be {\tt x0}; otherwise, {\em rs2} should be set to the ASID for which the translation is being modified. \item If software modifies a leaf PTE, it should execute SFENCE.VMA with {\em rs1} set to a virtual address within the page. If any PTE along the traversal path had its G bit set, {\em rs2} must be {\tt x0}; otherwise, {\em rs2} should be set to the ASID for which the translation is being modified. \item For the special cases of increasing the permissions on a leaf PTE and changing an invalid PTE to a valid leaf, software may choose to execute the SFENCE.VMA lazily. After modifying the PTE but before executing SFENCE.VMA, either the new or old permissions will be used. In the latter case, a page-fault exception might occur, at which point software should execute SFENCE.VMA in accordance with the previous bullet point. \end{itemize} \end{commentary} If a hart employs an address-translation cache, that cache must appear to be private to that hart. In particular, the meaning of an ASID is local to a hart; software may choose to use the same ASID to refer to different address spaces on different harts. \begin{commentary} A future extension could redefine ASIDs to be global across the SEE, enabling such options as shared translation caches and hardware support for broadcast TLB shootdown. However, as OSes have evolved to significantly reduce the scope of TLB shootdowns using novel ASID-management techniques, we expect the local-ASID scheme to remain attractive for its simplicity and possibly better scalability. \end{commentary} \section{Sv32: Page-Based 32-bit Virtual-Memory Systems} \label{sec:sv32} When Sv32 is written to the MODE field in the {\tt satp} register (see Section~\ref{sec:satp}), the supervisor operates in a 32-bit paged virtual-memory system. In this mode, supervisor and user virtual addresses are translated into supervisor physical addresses by traversing a radix-tree page table. Sv32 is supported on RV32 systems and is designed to include mechanisms sufficient for supporting modern Unix-based operating systems. \begin{commentary} The initial RISC-V paged virtual-memory architectures have been designed as straightforward implementations to support existing operating systems. We have architected page table layouts to support a hardware page-table walker. Software TLB refills are a performance bottleneck on high-performance systems, and are especially troublesome with decoupled specialized coprocessors. An implementation can choose to implement software TLB refills using a machine-mode trap handler as an extension to M-mode. \end{commentary} \begin{commentary} Some ISAs architecturally expose \emph{virtually indexed, physically tagged} caches, in that accesses to the same physical address via different virtual addresses might not be coherent unless the virtual addresses lie within the same cache set. Implicitly, this specification does not permit such behavior to be architecturally exposed. \end{commentary} For implementations that hardwire {\tt satp}.MODE to Bare, attempts to execute an SFENCE.VMA instruction might raise an illegal instruction exception. \subsection{Addressing and Memory Protection} \label{sec:translation} Sv32 implementations support a 32-bit virtual address space, divided into \wunits{4}{KiB} pages. An Sv32 virtual address is partitioned into a virtual page number (VPN) and page offset, as shown in Figure~\ref{sv32va}. When Sv32 virtual memory mode is selected in the MODE field of the {\tt satp} register, supervisor virtual addresses are translated into supervisor physical addresses via a two-level page table. The 20-bit VPN is translated into a 22-bit physical page number (PPN), while the 12-bit page offset is untranslated. The resulting supervisor-level physical addresses are then checked using any physical memory protection structures (Sections~\ref{sec:pmp}), before being directly converted to machine-level physical addresses. \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}O@{}O@{}E} \instbitrange{31}{22} & \instbitrange{21}{12} & \instbitrange{11}{0} \\ \hline \multicolumn{1}{|c|}{VPN[1]} & \multicolumn{1}{c|}{VPN[0]} & \multicolumn{1}{c|}{page offset} \\ \hline 10 & 10 & 12 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Sv32 virtual address.} \label{sv32va} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}E@{}O@{}E} \instbitrange{33}{22} & \instbitrange{21}{12} & \instbitrange{11}{0} \\ \hline \multicolumn{1}{|c|}{PPN[1]} & \multicolumn{1}{c|}{PPN[0]} & \multicolumn{1}{c|}{page offset} \\ \hline 12 & 10 & 12 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Sv32 physical address.} \label{rv32va} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}E@{}O@{}Fcccccccc} \instbitrange{31}{20} & \instbitrange{19}{10} & \instbitrange{9}{8} & \instbit{7} & \instbit{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{PPN[1]} & \multicolumn{1}{c|}{PPN[0]} & \multicolumn{1}{c|}{RSW} & \multicolumn{1}{c|}{D} & \multicolumn{1}{c|}{A} & \multicolumn{1}{c|}{G} & \multicolumn{1}{c|}{U} & \multicolumn{1}{c|}{X} & \multicolumn{1}{c|}{W} & \multicolumn{1}{c|}{R} & \multicolumn{1}{c|}{V} \\ \hline 12 & 10 & 2 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1\\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Sv32 page table entry.} \label{sv32pte} \end{figure*} Sv32 page tables consist of $2^{10}$ page-table entries (PTEs), each of four bytes. A page table is exactly the size of a page and must always be aligned to a page boundary. The physical page number of the root page table is stored in the {\tt satp} register. The PTE format for Sv32 is shown in Figures~\ref{sv32pte}. The V bit indicates whether the PTE is valid; if it is 0, all other bits in the PTE are don't-cares and may be used freely by software. The permission bits, R, W, and X, indicate whether the page is readable, writable, and executable, respectively. When all three are zero, the PTE is a pointer to the next level of the page table; otherwise, it is a leaf PTE. Writable pages must also be marked readable; the contrary combinations are reserved for future use. Table~\ref{pteperm} summarizes the encoding of the permission bits. \begin{table*}[h!] \begin{center} \begin{tabular}{|c|c|c||l|} \hline X & W & R & Meaning \\ \hline 0 & 0 & 0 & Pointer to next level of page table. \\ 0 & 0 & 1 & Read-only page. \\ 0 & 1 & 0 & {\em Reserved for future use.} \\ 0 & 1 & 1 & Read-write page. \\ 1 & 0 & 0 & Execute-only page. \\ 1 & 0 & 1 & Read-execute page. \\ 1 & 1 & 0 & {\em Reserved for future use.} \\ 1 & 1 & 1 & Read-write-execute page. \\ \hline \end{tabular} \end{center} \caption{Encoding of PTE R/W/X fields.} \label{pteperm} \end{table*} Attempting to fetch an instruction from a page that does not have execute permissions raises a fetch page-fault exception. Attempting to execute a load or load-reserved instruction whose effective address lies within a page without read permissions raises a load page-fault exception. Attempting to execute a store, store-conditional, or AMO instruction whose effective address lies within a page without write permissions raises a store page-fault exception. \begin{commentary} AMOs never raise load page-fault exceptions. Since any unreadable page is also unwritable, attempting to perform an AMO on an unreadable page always raises a store page-fault exception. \end{commentary} The U bit indicates whether the page is accessible to user mode. U-mode software may only access the page when U=1. If the SUM bit in the {\tt sstatus} register is set, supervisor mode software may also access pages with U=1. However, supervisor code normally operates with the SUM bit clear, in which case, supervisor code will fault on accesses to user-mode pages. Irrespective of SUM, the supervisor may not execute code on pages with U=1. \begin{commentary} An alternative PTE format would support different permissions for supervisor and user. We omitted this feature because it would be largely redundant with the SUM mechanism (see Section~\ref{sec:sum}) and would require more encoding space in the PTE. \end{commentary} The G bit designates a {\em global} mapping. Global mappings are those that exist in all address spaces. For non-leaf PTEs, the global setting implies that all mappings in the subsequent levels of the page table are global. Note that failing to mark a global mapping as global merely reduces performance, whereas marking a non-global mapping as global is a software bug that, after switching to an address space with a different non-global mapping for that address range, can unpredictably result in either mapping being used. \begin{commentary} Global mappings need not be stored redundantly in address-translation caches for multiple ASIDs. Additionally, they need not be flushed from local address-translation caches when an SFENCE.VMA instruction is executed with {\em rs2}$\neq${\tt x0}. \end{commentary} The RSW field is reserved for use by supervisor software; the implementation shall ignore this field. Each leaf PTE contains an accessed (A) and dirty (D) bit. The A bit indicates the virtual page has been read, written, or fetched from since the last time the A bit was cleared. The D bit indicates the virtual page has been written since the last time the D bit was cleared. Two schemes to manage the A and D bits are permitted: \begin{itemize} \item When a virtual page is accessed and the A bit is clear, or is written and the D bit is clear, a page-fault exception is raised. \item When a virtual page is accessed and the A bit is clear, or is written and the D bit is clear, the implementation sets the corresponding bit(s) in the PTE. The PTE update must be atomic with respect to other accesses to the PTE, and must atomically check that the PTE is valid and grants sufficient permissions. The PTE update must be exact (i.e., not speculative), and observed in program order by the local hart. Furthermore, the PTE update must appear in the global memory order no later than the explicit memory access, or any subsequent explicit memory access to that virtual page by the local hart. The ordering on loads and stores provided by FENCE instructions and the acquire/release bits on atomic instructions also orders the PTE updates associated with those loads and stores as observed by remote harts. The PTE update is not required to be atomic with respect to the explicit memory access that caused the update, and the sequence is interruptible. However, the hart must not perform the explicit memory access before the PTE update is globally visible. \end{itemize} All harts in a system must employ the same PTE-update scheme as each other. \begin{commentary} Mandating that the PTE updates to be exact, atomic, and in program order simplifies the specification, and makes the feature more useful for system software. Simple implementations may instead generate page-fault exceptions. The A and D bits are never cleared by the implementation. If the supervisor software does not rely on accessed and/or dirty bits, e.g. if it does not swap memory pages to secondary storage or if the pages are being used to map I/O space, it should always set them to 1 in the PTE to improve performance. \end{commentary} Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv32 supports 4 MiB {\em megapages}. A megapage must be virtually and physically aligned to a 4 MiB boundary; a page-fault exception is raised if the physical address is insufficiently aligned. For non-leaf PTEs, the D, A, and U bits are reserved for future standard use and must be cleared by software for forward compatibility. For implementations with both page-based virtual memory and the ``A'' standard extension, the LR/SC reservation set must lie completely within a single base page (i.e., a naturally aligned \wunits{4}{KiB} region). \subsection{Virtual Address Translation Process} \label{sv32algorithm} A virtual address $va$ is translated into a physical address $pa$ as follows: \begin{enumerate} \item Let $a$ be ${\tt satp}.ppn \times \textrm{PAGESIZE}$, and let $i=\textrm{LEVELS} - 1$. (For Sv32, PAGESIZE=$2^{12}$ and LEVELS=2.) \item Let $pte$ be the value of the PTE at address $a+va.vpn[i]\times \textrm{PTESIZE}$. (For Sv32, PTESIZE=4.) If accessing $pte$ violates a PMA or PMP check, raise an access-fault exception corresponding to the original access type. \item If $pte.v=0$, or if $pte.r=0$ and $pte.w=1$, stop and raise a page-fault exception corresponding to the original access type. \item Otherwise, the PTE is valid. If $pte.r=1$ or $pte.x=1$, go to step 5. Otherwise, this PTE is a pointer to the next level of the page table. Let $i=i-1$. If $i<0$, stop and raise a page-fault exception corresponding to the original access type. Otherwise, let $a=pte.ppn \times \textrm{PAGESIZE}$ and go to step 2. \item A leaf PTE has been found. Determine if the requested memory access is allowed by the $pte.r$, $pte.w$, $pte.x$, and $pte.u$ bits, given the current privilege mode and the value of the SUM and MXR fields of the {\tt mstatus} register. If not, stop and raise a page-fault exception corresponding to the original access type. \item If $i>0$ and $pte.ppn[i-1:0]\neq 0$, this is a misaligned superpage; stop and raise a page-fault exception corresponding to the original access type. \item If $pte.a=0$, or if the memory access is a store and $pte.d=0$, either raise a page-fault exception corresponding to the original access type, or: \begin{itemize} \item Set $pte.a$ to 1 and, if the memory access is a store, also set $pte.d$ to 1. \item If this access violates a PMA or PMP check, raise an access-fault exception corresponding to the original access type. \item This update and the loading of $pte$ in step 2 must be atomic; in particular, no intervening store to the PTE may be perceived to have occurred in-between. \end{itemize} \item The translation is successful. The translated physical address is given as follows: \begin{itemize} \item $\textit{pa.pgoff} = \textit{va.pgoff}$. \item If $i>0$, then this is a superpage translation and $pa.ppn[i-1:0]=va.vpn[i-1:0]$. \item $pa.ppn[\textrm{LEVELS} - 1:i] = pte.ppn[\textrm{LEVELS} - 1:i]$. \end{itemize} \end{enumerate} \section{Sv39: Page-Based 39-bit Virtual-Memory System} \label{sec:sv39} This section describes a simple paged virtual-memory system designed for RV64 systems, which supports 39-bit virtual address spaces. The design of Sv39 follows the overall scheme of Sv32, and this section details only the differences between the schemes. \begin{commentary} We specified multiple virtual memory systems for RV64 to relieve the tension between providing a large address space and minimizing address-translation cost. For many systems, \wunits{512}{GiB} of virtual-address space is ample, and so Sv39 suffices. Sv48 increases the virtual address space to \wunits{256}{TiB}, but increases the physical memory capacity dedicated to page tables, the latency of page-table traversals, and the size of hardware structures that store virtual addresses. \end{commentary} \subsection{Addressing and Memory Protection} Sv39 implementations support a 39-bit virtual address space, divided into \wunits{4}{KiB} pages. An Sv39 address is partitioned as shown in Figure~\ref{sv39va}. Instruction fetch addresses and load and store effective addresses, which are 64 bits, must have bits 63--39 all equal to bit 38, or else a page-fault exception will occur. The 27-bit VPN is translated into a 44-bit PPN via a three-level page table, while the 12-bit page offset is untranslated. \begin{commentary} When mapping between narrower and wider addresses, RISC-V usually zero-extends a narrower address to a wider size. The mapping between 64-bit virtual addresses and the 39-bit usable address space of Sv39 is not based on zero-extension but instead follows an entrenched convention that allows an OS to use one or a few of the most-significant bits of a full-size (64-bit) virtual address to quickly distinguish user and supervisor address regions. \end{commentary} \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}O@{}O@{}O@{}O} \instbitrange{38}{30} & \instbitrange{29}{21} & \instbitrange{20}{12} & \instbitrange{11}{0} \\ \hline \multicolumn{1}{|c|}{VPN[2]} & \multicolumn{1}{c|}{VPN[1]} & \multicolumn{1}{c|}{VPN[0]} & \multicolumn{1}{c|}{page offset} \\ \hline 9 & 9 & 9 & 12 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Sv39 virtual address.} \label{sv39va} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}T@{}O@{}O@{}O} \instbitrange{55}{30} & \instbitrange{29}{21} & \instbitrange{20}{12} & \instbitrange{11}{0} \\ \hline \multicolumn{1}{|c|}{PPN[2]} & \multicolumn{1}{c|}{PPN[1]} & \multicolumn{1}{c|}{PPN[0]} & \multicolumn{1}{c|}{page offset} \\ \hline 26 & 9 & 9 & 12 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Sv39 physical address.} \label{sv39pa} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}Y@{}Y@{}Y@{}Y@{}Fcccccccc} \instbitrange{63}{54} & \instbitrange{53}{28} & \instbitrange{27}{19} & \instbitrange{18}{10} & \instbitrange{9}{8} & \instbit{7} & \instbit{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{\it Reserved} & \multicolumn{1}{c|}{PPN[2]} & \multicolumn{1}{c|}{PPN[1]} & \multicolumn{1}{c|}{PPN[0]} & \multicolumn{1}{c|}{RSW} & \multicolumn{1}{c|}{D} & \multicolumn{1}{c|}{A} & \multicolumn{1}{c|}{G} & \multicolumn{1}{c|}{U} & \multicolumn{1}{c|}{X} & \multicolumn{1}{c|}{W} & \multicolumn{1}{c|}{R} & \multicolumn{1}{c|}{V} \\ \hline 10 & 26 & 9 & 9 & 2 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1\\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Sv39 page table entry.} \label{sv39pte} \end{figure*} Sv39 page tables contain $2^9$ page table entries (PTEs), eight bytes each. A page table is exactly the size of a page and must always be aligned to a page boundary. The physical page number of the root page table is stored in the {\tt satp} register's PPN field. The PTE format for Sv39 is shown in Figure~\ref{sv39pte}. Bits 9--0 have the same meaning as for Sv32. Bits 63--54 are reserved for future standard use and must be zeroed by software for forward compatibility. \begin{commentary} We reserved several PTE bits for a possible extension that improves support for sparse address spaces by allowing page-table levels to be skipped, reducing memory usage and TLB refill latency. These reserved bits may also be used to facilitate research experimentation. The cost is reducing the physical address space, but \wunits{64}{PiB} is presently ample. When it no longer suffices, the reserved bits that remain unallocated could be used to expand the physical address space. \end{commentary} Any level of PTE may be a leaf PTE, so in addition to \wunits{4}{KiB} pages, Sv39 supports \wunits{2}{MiB} {\em megapages} and \wunits{1}{GiB} {\em gigapages}, each of which must be virtually and physically aligned to a boundary equal to its size. A page-fault exception is raised if the physical address is insufficiently aligned. The algorithm for virtual-to-physical address translation is the same as in Section~\ref{sv32algorithm}, except LEVELS equals 3 and PTESIZE equals 8. \section{Sv48: Page-Based 48-bit Virtual-Memory System} \label{sec:sv48} This section describes a simple paged virtual-memory system designed for RV64 systems, which supports 48-bit virtual address spaces. Sv48 is intended for systems for which a 39-bit virtual address space is insufficient. It closely follows the design of Sv39, simply adding an additional level of page table, and so this chapter only details the differences between the two schemes. Implementations that support Sv48 must also support Sv39. \begin{commentary} Systems that support Sv48 can also support Sv39 at essentially no cost, and so should do so to maintain compatibility with supervisor software that assumes Sv39. \end{commentary} \subsection{Addressing and Memory Protection} Sv48 implementations support a 48-bit virtual address space, divided into \wunits{4}{KiB} pages. An Sv48 address is partitioned as shown in Figure~\ref{sv48va}. Instruction fetch addresses and load and store effective addresses, which are 64 bits, must have bits 63--48 all equal to bit 47, or else a page-fault exception will occur. The 36-bit VPN is translated into a 44-bit PPN via a four-level page table, while the 12-bit page offset is untranslated. \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}O@{}O@{}O@{}O@{}O} \instbitrange{47}{39} & \instbitrange{38}{30} & \instbitrange{29}{21} & \instbitrange{20}{12} & \instbitrange{11}{0} \\ \hline \multicolumn{1}{|c|}{VPN[3]} & \multicolumn{1}{c|}{VPN[2]} & \multicolumn{1}{c|}{VPN[1]} & \multicolumn{1}{c|}{VPN[0]} & \multicolumn{1}{c|}{page offset} \\ \hline 9 & 9 & 9 & 9 & 12 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Sv48 virtual address.} \label{sv48va} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}E@{}O@{}O@{}O@{}O} \instbitrange{55}{39} & \instbitrange{38}{30} & \instbitrange{29}{21} & \instbitrange{20}{12} & \instbitrange{11}{0} \\ \hline \multicolumn{1}{|c|}{PPN[3]} & \multicolumn{1}{c|}{PPN[2]} & \multicolumn{1}{c|}{PPN[1]} & \multicolumn{1}{c|}{PPN[0]} & \multicolumn{1}{c|}{page offset} \\ \hline 17 & 9 & 9 & 9 & 12 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Sv48 physical address.} \label{sv48pa} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}Y@{}Y@{}Y@{}Y@{}Y@{}Fcccccccc} \instbitrange{63}{54} & \instbitrange{53}{37} & \instbitrange{36}{28} & \instbitrange{27}{19} & \instbitrange{18}{10} & \instbitrange{9}{8} & \instbit{7} & \instbit{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{\it Reserved} & \multicolumn{1}{c|}{PPN[3]} & \multicolumn{1}{c|}{PPN[2]} & \multicolumn{1}{c|}{PPN[1]} & \multicolumn{1}{c|}{PPN[0]} & \multicolumn{1}{c|}{RSW} & \multicolumn{1}{c|}{D} & \multicolumn{1}{c|}{A} & \multicolumn{1}{c|}{G} & \multicolumn{1}{c|}{U} & \multicolumn{1}{c|}{X} & \multicolumn{1}{c|}{W} & \multicolumn{1}{c|}{R} & \multicolumn{1}{c|}{V} \\ \hline 10 & 17 & 9 & 9 & 9 & 2 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1\\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Sv48 page table entry.} \label{sv48pte} \end{figure*} The PTE format for Sv48 is shown in Figure~\ref{sv48pte}. Bits 9--0 have the same meaning as for Sv32. Any level of PTE may be a leaf PTE, so in addition to \wunits{4}{KiB} pages, Sv48 supports \wunits{2}{MiB} {\em megapages}, \wunits{1}{GiB} {\em gigapages}, and \wunits{512}{GiB} {\em terapages}, each of which must be virtually and physically aligned to a boundary equal to its size. A page-fault exception is raised if the physical address is insufficiently aligned. The algorithm for virtual-to-physical address translation is the same as in Section~\ref{sv32algorithm}, except LEVELS equals 4 and PTESIZE equals 8.