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author | Andrew Waterman <andrew@sifive.com> | 2017-11-09 11:02:12 -0800 |
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committer | Andrew Waterman <andrew@sifive.com> | 2017-11-09 11:02:12 -0800 |
commit | 6a92b89bdf7d8d83b6e4a828658628c991aeab50 (patch) | |
tree | dc893072415368d92ae02a58fd67cf6eefca0751 /src/hypervisor.tex | |
parent | 14e18f2036c82943e52ca8bf630fd8e4f53b7e4a (diff) | |
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Add hypervisor draft proposal
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1 files changed, 1095 insertions, 4 deletions
diff --git a/src/hypervisor.tex b/src/hypervisor.tex index e054bdd..05aa401 100644 --- a/src/hypervisor.tex +++ b/src/hypervisor.tex @@ -1,11 +1,1102 @@ -\chapter{Hypervisor Extensions, Version 0.0} +\chapter{Hypervisor Extension, Version 0.1-draft} \label{hypervisor} -This chapter is a placeholder for RISC-V hypervisor support with an -extended S-mode. +This chapter describes the RISC-V hypervisor extension, which virtualizes the +supervisor-level architecture to support the efficient hosting of guest +operating systems atop a type-1 or type-2 hypervisor. +The hypervisor extension adds a new privilege +mode, {\em hypervisor-extended supervisor mode} (HS-mode, or {\em hypervisor +mode} for short), where a hypervisor or a hosting-capable operating system +runs. The hypervisor extension also adds another level of address translation, +from guest virtual addresses to host virtual addresses, to virtualize the +memory and memory-mapped I/O subsystems for a guest operating system. HS-mode +acts the same as S-mode, but with additional instructions and CSRs that control +the new level of address translation and support hosting an S-mode guest. +Regular S-mode operating systems can execute without modification either in +HS-mode or as S-mode guests. + +In HS-mode, an OS or hypervisor interacts with the machine through the same +SBI as an OS normally does from S-mode. An HS-mode hypervisor is expected to +implement the SBI for its S-mode guest. + +The hypervisor extension is enabled by setting bit 7 in the {\tt misa} CSR, +which corresponds to the letter H. When {\tt misa}[7] is clear, the hart +behaves as though this extension were not implemented, and attempts to use +hypervisor CSRs or instructions raise an illegal instruction exception. +Implementations that include the hypervisor extension are encouraged +not to hardwire {\tt misa}[7], so that the extension may be disabled. + +\begin{commentary} +This draft is based on earlier proposals by John Hauser and Paolo Bonzini. +\end{commentary} \begin{commentary} -The privileged architecture is designed to simplify the use of classic +The baseline privileged architecture is designed to simplify the use of classic virtualization techniques, where a guest OS is run at user-level, as the few privileged instructions can be easily detected and trapped. +The hypervisor extension improves virtualization performance by +reducing the frequency of these traps. + +The hypervisor extension has been designed to be efficiently +emulable on platforms that do not implement the extension, by running +the hypervisor in S-mode and trapping into M-mode for hypervisor CSR accesses +and to maintain shadow page tables. The majority of CSR accesses for +type-2 hypervisors are valid S-mode accesses so need not be trapped. +Hypervisors can support nested virtualization analogously. +\end{commentary} + +\section{Privilege Modes} + +The current {\em virtualization mode}, denoted V, indicates whether the hart +is currently executing in a guest. When V=1, the hart is either in S-mode, or +in U-mode under an OS running as an S-mode guest. When V=0, the hart is +either in M-mode, in HS-mode, or in U-mode under an OS running in HS-mode. +The virtualization mode also indicates whether two-level address translation +is active (V=1) or inactive (V=0). Table~\ref{h-operating-modes} lists the +possible operating modes of a RISC-V hart with the hypervisor extension. + +\begin{table*}[h!] +\begin{center} +\begin{tabular}{|c|c||l|l|l|} + \hline + Virtualization & Privilege & \multirow{2}{*}{Abbreviation} & \multirow{2}{*}{Name} & Two-Level \\ + Mode (V) & Encoding & & & Translation \\ \hline + 0 & 0 & U-mode & User mode & Off \\ + 0 & 1 & HS-mode & Hypervisor-extended supervisor mode & Off \\ + 0 & 3 & M-mode & Machine mode & Off \\ + \hline + 1 & 0 & U-mode & User mode & On \\ + 1 & 1 & S-mode & Supervisor mode & On \\ + \hline + \end{tabular} +\end{center} +\caption{Operating modes with the hypervisor extension.} +\label{h-operating-modes} +\end{table*} + +\section{Hypervisor CSRs} + +An OS or hypervisor running in HS-mode uses the supervisor CSRs to interact with the exception, +interrupt, and address-translation subsystems. +Additional CSRs are provided to HS-mode, but not to S-mode, to control +the behavior of an S-mode guest: +{\tt hvtval}, {\tt hstatus}, {\tt hedeleg}, and +{\tt hideleg}. + +Additionally, several {\em background} supervisor CSRs are copies of one of +the existing {\em foreground} supervisor CSRs. For example, the {\tt +bsstatus} CSR is the background copy of the foreground {\tt sstatus} CSR. +When transitioning between virtualization modes (V=0 to V=1, or vice-versa), +the implementation swaps the background supervisor CSRs with their foreground +counterparts. When V=0, the background supervisor CSRs contain the S-mode +guest's version of those CSRs, and the foreground supervisor CSRs contain +HS-mode's version. When V=1, the background supervisor CSRs contain HS-mode's +version, and the foreground supervisor CSRs contain the S-mode guest's +version. The background registers are accessible to HS-mode, but not to S-mode. + +In this section, we use the term {\em HS-XLEN} to refer to the effective XLEN +when executing in HS-mode. + +\subsection{Hypervisor Virtual Trap Value ({\tt hvtval}) Register} + +The {\tt hvtval} register is an XLEN-bit read-write register formatted as shown +in Figure~\ref{hvtvalreg}. When an access fault, page fault, or misaligned +address exception is taken into HS-mode, {\tt hvtval} is +written with the original virtual address that caused the exception. +For other traps into HS-mode, {\tt hvtval} is written with zero. + +\begin{figure}[h!] +{\footnotesize +\begin{center} +\begin{tabular}{@{}J} +\instbitrange{XLEN-1}{0} \\ +\hline +\multicolumn{1}{|c|}{\tt hvtval} \\ +\hline +XLEN \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Hypervisor virtual trap value register ({\tt hvtval}).} +\label{hvtvalreg} +\end{figure} + +\subsection{Hypervisor Status ({\tt hstatus}) Register} + +The {\tt hstatus} register is an XLEN-bit read/write register +formatted as shown in Figure~\ref{hstatusreg}. The {\tt hstatus} +register provides facilities analogous to the {\tt mstatus} register +that track and control the exception behavior of an S-mode guest. + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\setlength{\tabcolsep}{4pt} +\begin{tabular}{EcccWcYccR} +\\ +\instbitrange{XLEN-1}{23} & +\instbit{22} & +\instbit{21} & +\instbit{20} & +\instbitrange{19}{18} & +\instbit{17} & +\instbitrange{16}{11} & +\instbit{10} & +\instbit{9} & +\instbitrange{8}{0} \\ +\hline +\multicolumn{1}{|c|}{\wpri} & +\multicolumn{1}{c|}{VTSR} & +\multicolumn{1}{c|}{VTW} & +\multicolumn{1}{c|}{VTVM} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{SPRV} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{SPV} & +\multicolumn{1}{c|}{STL} & +\multicolumn{1}{c|}{\wpri} \\ +\hline +XLEN-23 & 1 & 1 & 1 & 2 & 1 & 6 & 1 & 1 & 9 \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Hypervisor-mode status register ({\tt hstatus}).} +\label{hstatusreg} +\end{figure*} + +The {\tt hstatus} fields VTSR, VTW, and VTVM are defined analogously to the +{\tt mstatus} fields TSR, TW, and TVM, but affect the trapping behavior of the +SRET, WFI, and virtual-memory management instructions in the S-mode guest +only. + +The SPV bit (Supervisor Previous Virtualization Mode) is written by the implementation +whenever a trap is taken into HS-mode. Just as the SPP bit in {\tt sstatus} is set to the privilege +mode at the time of the trap, the SPV bit in {\tt hstatus} is set to the value of the virtualization +mode V at the time of the trap. When an SRET instruction is executed when V=0, +V is set to SPV. + +The STL bit (Supervisor Translation Level), which indicates which address-translation level +caused a page-fault exception, is also written by the implementation whenever a trap +is taken into HS-mode. On an access fault, or on a page fault due to HS-level address +translation, STL is set to 0. For any other trap into HS-mode, STL is set to the value +of V at the time of the trap. + +The SPRV bit modifies the privilege with which loads and stores execute. +When SPRV=0, translation and protection behave as normal. When SPRV=1, +load and store memory addresses are translated and protected as though +the current privilege mode were set to {\tt sstatus}.SPP and the current +virtualization mode were set to {\tt hstatus}.SPV. +Table~\ref{h-sprv} enumerates the cases. + +\begin{table*}[h!] +\begin{center} +\begin{tabular}{|c|c|c||p{5in}|} + \hline + SPRV & SPV & SPP & Effect \\ \hline \hline + 0 & -- & -- & Normal access; current privilege and virtualization modes apply. \\ \hline + 1 & 0 & 0 & U-level access with HS-level translation and protection only. \\ \hline + 1 & 0 & 1 & HS-level access with HS-level translation and protection only. \\ \hline + 1 & 1 & 0 & U-level access with two-level translation and protection. {\tt sstatus}.MXR makes any executable page readable. {\tt bsstatus}.MXR makes readable those pages marked executable at the S translation level only if readable at the HS translation level. \\ \hline + 1 & 1 & 1 & S-level access with two-level translation and protection. {\tt sstatus}.MXR makes any executable page readable. {\tt bsstatus}.MXR makes readable those pages marked executable at the S translation level only if readable at the HS translation level. {\tt bsstatus}.SUM applies instead of {\tt sstatus}.SUM. \\ \hline + \end{tabular} +\end{center} +\caption{Effect on load and store translation and protection under SPRV.} +\label{h-sprv} +\end{table*} + +\begin{commentary} +For simplicity, SPRV is in effect even when in S-mode or U-mode, but in normal +use will only be enabled for short sequences in HS-mode. +\end{commentary} + +\subsection{Hypervisor Trap Delegation Registers ({\tt hedeleg} and {\tt hideleg})} + +By default, all traps at any privilege level are handled in M-mode, though +M-mode usually uses the {\tt medeleg} and {\tt mideleg} CSRs to delegate +some traps to HS-mode. The {\tt hedeleg} and {\tt hideleg} CSRs allow these +traps to be further delegated to an S-mode guest; their layout is the same +as {\tt medeleg} and {\tt mideleg}. + +\begin{figure}[h!] +{\footnotesize +\begin{center} +\begin{tabular}{@{}U} +\instbitrange{XLEN-1}{0} \\ +\hline +\multicolumn{1}{|c|}{Synchronous Exceptions (\warl)} \\ +\hline +XLEN \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Hypervisor Exception Delegation Register {\tt hedeleg}.} +\label{hedelegreg} +\end{figure} + +\begin{figure}[h!] +{\footnotesize +\begin{center} +\begin{tabular}{@{}U} +\instbitrange{XLEN-1}{0} \\ +\hline +\multicolumn{1}{|c|}{Interrupts (\warl)} \\ +\hline +XLEN \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Hypervisor Interrupt Delegation Register {\tt hideleg}.} +\label{hidelegreg} +\end{figure} + +The {\tt hedeleg} and {\tt hideleg} registers are only active when V=1. When +V=1, any trap that has been delegated to HS-mode (using {\tt medeleg} or {\tt +mideleg}) is further delegated to S-mode if the corresponding {\tt hedeleg} or +{\tt hideleg} bit is set. If the N extension for user-mode interrupts +is implemented, the S-mode guest may further delegate the interrupt +to U-mode by setting the corresponding bit in {\tt sedeleg} or {\tt sideleg}. + +When V=0 and the N extension for user-mode interrupts is implemented, any trap +that has been delegated to HS-mode can be further delegated to U-mode by +setting the corresponding bit in {\tt sedeleg} or {\tt sideleg}. + +\subsection{Background Supervisor Status ({\tt bsstatus}) Register} + +The {\tt bsstatus} register is an XLEN-bit read/write register formatted as +shown in Figure~\ref{bsstatusreg}. When V=0, the {\tt bsstatus} register +holds the S-mode guest's version of several fields of the {\tt sstatus} +register: UXL, MXR, SUM, FS, SPP, SPIE, and SIE. When V=1, {\tt bsstatus} +holds HS-mode's version of these fields. When transitioning between +virtualization modes (V=0 to V=1, or vice-versa), the implementation swaps +these fields in {\tt bsstatus} with their counterparts in {\tt sstatus}. The +other fields in {\tt sstatus} are unchanged. + +When V=1, both {\tt bsstatus}.FS and {\tt sstatus}.FS are in effect. Attempts +to execute a floating-point instruction when either field is 0 (Off) raise an +illegal-instruction exception. Modifying the floating-point state when V=1 +causes both fields to be set to 3 (Dirty). + +When V=0, {\tt bsstatus} does not directly affect the behavior of the machine, +unless the MPRV feature in the {\tt mstatus} register or the SPRV feature +in the {\tt hstatus} register is used to execute a load or store +{\em as though} V=1. + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\setlength{\tabcolsep}{4pt} +\begin{tabular}{McEccc} +\\ +\instbitrange{XLEN-1}{34} & +\instbitrange{33}{32} & +\instbitrange{31}{20} & +\instbit{19} & +\instbit{18} & + \\ +\hline +\multicolumn{1}{|c|}{\wpri} & +\multicolumn{1}{c|}{UXL} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{MXR} & +\multicolumn{1}{c|}{SUM} & + \\ +\hline +XLEN-34 & 2 & 12 & 1 & 1 & \\ +\end{tabular} +\begin{tabular}{cFFYcWcFcc} +\\ +& +\instbitrange{17}{15} & +\instbitrange{14}{13} & +\instbitrange{12}{9} & +\instbit{8} & +\instbitrange{7}{6} & +\instbit{5} & +\instbitrange{4}{2} & +\instbit{1} & +\instbit{0} \\ +\hline + & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{FS[1:0]} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{SPP} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{SPIE} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{SIE} & +\multicolumn{1}{c|}{\wpri} \\ +\hline + & 3 & 2 & 4 & 1 & 2 & 1 & 3 & 1 & 1 \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Background supervisor status register ({\tt bsstatus}) for RV64 and RV128.} +\label{bsstatusreg} +\end{figure*} + +\subsection{Background Supervisor Interrupt Registers ({\tt bsip} and {\tt bsie})} + +The {\tt bsip} register is an XLEN-bit read/write register formatted as shown +in Figure~\ref{bsipreg}. When V=0, the {\tt bsip} register holds the S-mode +guest's version of the {\tt sip} register. When V=1, {\tt bsip} holds +HS-mode's version of the {\tt sip} register. When transitioning between +virtualization modes (V=0 to V=1, or vice-versa), the implementation swaps the +defined fields of {\tt bsip} with their counterparts in {\tt sip}. The +other fields in {\tt sip} are unchanged. + +\note{AW: Need to describe how {\tt bsip}.SEIP interacts with PLIC. I think {\tt bsip}.SEIP should purely be a read-write storage bit to emulate the PLIC for S-mode; the PLIC should not be wired into {\tt bsip}.SEIP.} + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\setlength{\tabcolsep}{4pt} +\begin{tabular}{TcFcFcc} +\instbitrange{XLEN-1}{10} & +\instbit{9} & +\instbitrange{8}{6} & +\instbit{5} & +\instbitrange{4}{2} & +\instbit{1} & +\instbit{0} \\ +\hline +\multicolumn{1}{|c|}{\wiri} & +\multicolumn{1}{c|}{SEIP} & +\multicolumn{1}{c|}{\wiri} & +\multicolumn{1}{c|}{STIP} & +\multicolumn{1}{c|}{\wiri} & +\multicolumn{1}{c|}{SSIP} & +\multicolumn{1}{c|}{\wiri} \\ +\hline +XLEN-10 & 1 & 3 & 1 & 3 & 1 & 1 \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Background supervisor interrupt-pending register ({\tt bsip}).} +\label{bsipreg} +\end{figure*} + +The {\tt bsie} register is an XLEN-bit read/write register formatted as shown +in Figure~\ref{bsiereg}. When V=0, the {\tt bsie} register holds the S-mode +guest's version of the {\tt sie} register. When V=1, {\tt bsie} holds +HS-mode's version of the {\tt sie} register. When transitioning between +virtualization modes (V=0 to V=1, or vice-versa), the implementation swaps the +defined fields of {\tt bsie} with their counterparts in {\tt sie}. The +other fields in {\tt sie} are unchanged. + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\setlength{\tabcolsep}{4pt} +\begin{tabular}{TcFcFcc} +\instbitrange{XLEN-1}{10} & +\instbit{9} & +\instbitrange{8}{6} & +\instbit{5} & +\instbitrange{4}{2} & +\instbit{1} & +\instbit{0} \\ +\hline +\multicolumn{1}{|c|}{\wpri} & +\multicolumn{1}{c|}{SEIE} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{STIE} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{SSIE} & +\multicolumn{1}{c|}{\wiri} \\ +\hline +XLEN-10 & 1 & 3 & 1 & 3 & 1 & 1 \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Background supervisor interrupt-enable register ({\tt bsie}).} +\label{bsiereg} +\end{figure*} + +When V=0, {\tt bsip} and {\tt bsie} do not affect the behavior of the machine. +When V=1, they hold the active interrupt-pending and interrupt-enable bits, +respectively, for HS-mode; if any bit position holds a 1 in both registers, an +interrupt will be taken. + +\begin{commentary} +The {\tt bsip} and {\tt bsie} CSRs do not hold copies of the user-mode +interrupt fields. The expectation is that the context-switch code +will swap the {\tt uip} and {\tt uie} CSRs +along with the other user-mode interrupt +registers ({\tt ustatus}, {\tt utvec}, etc.) if that feature is enabled. +\end{commentary} + +\subsection{Background Supervisor Trap Vector Base Address Register ({\tt bstvec})} + +The {\tt bstvec} register is an XLEN-bit read/write register formatted as shown +in Figure~\ref{bstvecreg}. When V=0, the {\tt bstvec} register holds the +S-mode guest's version of the {\tt stvec} register. When V=1, {\tt bstvec} +holds HS-mode's version of the {\tt stvec} register. When transitioning between +virtualization modes (V=0 to V=1, or vice-versa), the implementation swaps the +contents of {\tt bstvec} and {\tt stvec}. + +When V=0, {\tt bstvec} does not directly affect the behavior of the machine. When V=1, +it controls the value to which the {\tt pc} will be set upon a trap into +HS-mode. + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\begin{tabular}{J@{}R} +\instbitrange{XLEN-1}{2} & +\instbitrange{1}{0} \\ +\hline +\multicolumn{1}{|c|}{BASE[XLEN-1:2] (\warl)} & +\multicolumn{1}{c|}{MODE (\warl)} \\ +\hline +XLEN-2 & 2 \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Background supervisor trap vector base address register ({\tt bstvec}).} +\label{bstvecreg} +\end{figure*} + +\subsection{Background Supervisor Scratch Register ({\tt bsscratch})} + +The {\tt bsscratch} register is an XLEN-bit read/write register formatted as shown +in Figure~\ref{bsscratchreg}. When V=0, the {\tt bsscratch} register holds the +S-mode guest's version of the {\tt sscratch} register. When V=1, {\tt bsscratch} +holds HS-mode's version of the {\tt sscratch} register. When transitioning between +virtualization modes (V=0 to V=1, or vice-versa), the implementation swaps the +contents of {\tt bsscratch} and {\tt sscratch}. + +Typically, {\tt bsscratch} is used to hold a pointer to the hart-local +hypervisor context (when V=1) or supervisor context (when V=0). The +contents of {\tt bsscratch} do not directly affect the behavior of +the machine. + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\begin{tabular}{@{}J} +\instbitrange{XLEN-1}{0} \\ +\hline +\multicolumn{1}{|c|}{\tt bsscratch} \\ +\hline +XLEN \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Background supervisor scratch register ({\tt bsscratch}).} +\label{bsscratchreg} +\end{figure*} + +\subsection{Background Supervisor Exception Program Counter ({\tt bsepc})} + +The {\tt bsepc} register is an XLEN-bit read/write register formatted as shown +in Figure~\ref{bsepcreg}. When V=0, the {\tt bsepc} register holds the +S-mode guest's version of the {\tt sepc} register. When V=1, {\tt bsepc} +holds HS-mode's version of the {\tt sepc} register. When transitioning between +virtualization modes (V=0 to V=1, or vice-versa), the implementation swaps the +contents of {\tt bsepc} and {\tt sepc}. + +The contents of {\tt bsepc} do not directly affect the behavior of +the machine. + +{\tt bsepc} is a \warl\ register that must be able to hold the same set of +values that {\tt sepc} can hold. + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\begin{tabular}{@{}J} +\instbitrange{XLEN-1}{0} \\ +\hline +\multicolumn{1}{|c|}{\tt bsepc} \\ +\hline +XLEN \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Background supervisor exception program counter ({\tt bsepc}).} +\label{bsepcreg} +\end{figure*} + +\subsection{Background Supervisor Cause Register ({\tt bscause})} + +The {\tt bscause} register is an XLEN-bit read/write register formatted as shown +in Figure~\ref{bscausereg}. When V=0, the {\tt bscause} register holds the +S-mode guest's version of the {\tt scause} register. When V=1, {\tt bscause} +holds HS-mode's version of the {\tt scause} register. When transitioning between +virtualization modes (V=0 to V=1, or vice-versa), the implementation swaps the +contents of {\tt bscause} and {\tt scause}. + +The contents of {\tt bscause} do not directly affect the behavior of +the machine. + +{\tt bscause} is a \wlrl\ register that must be able to hold the same set of +values that {\tt scause} can hold. + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\begin{tabular}{c@{}U} +\instbit{XLEN-1} & +\instbitrange{XLEN-2}{0} \\ +\hline +\multicolumn{1}{|c|}{Interrupt} & +\multicolumn{1}{c|}{Exception Code (\wlrl)} \\ +\hline +1 & XLEN-1 \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Background supervisor cause register ({\tt bscause}).} +\label{bscausereg} +\end{figure*} + +\subsection{Background Supervisor Trap Value Register ({\tt bstval})} + +The {\tt bstval} register is an XLEN-bit read/write register formatted as shown +in Figure~\ref{bstvalreg}. When V=0, the {\tt bstval} register holds the +S-mode guest's version of the {\tt stval} register. When V=1, {\tt bstval} +holds HS-mode's version of the {\tt stval} register. When transitioning between +virtualization modes (V=0 to V=1, or vice-versa), the implementation swaps the +contents of {\tt bstval} and {\tt stval}. + +The contents of {\tt bstval} do not directly affect the behavior of +the machine. + +{\tt bstval} is a \warl\ register that must be able to hold the same set of +values that {\tt stval} can hold. + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\begin{tabular}{@{}J} +\instbitrange{XLEN-1}{0} \\ +\hline +\multicolumn{1}{|c|}{\tt bstval} \\ +\hline +XLEN \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Background supervisor trap value register ({\tt bstval}).} +\label{bstvalreg} +\end{figure*} + +\subsection{Background Supervisor Address Translation and Protection Register ({\tt bsatp})} + +The {\tt bsatp} register is an XLEN-bit read/write register formatted as shown +in Figure~\ref{rv32bsatpreg} for RV32 and Figure~\ref{rv64bsatpreg} for RV64. +When V=0, the {\tt bsatp} register holds the S-mode guest's version of the +{\tt satp} register. When V=1, {\tt bsatp} holds HS-mode's version of the +{\tt satp} register. When transitioning between virtualization modes (V=0 to +V=1, or vice-versa), the implementation swaps the contents of {\tt bsatp} and +{\tt satp}. + +When V=0, {\tt bsatp} does not directly affect the behavior of the machine. When V=1, +it controls the translation of guest physical addresses to +machine physical addresses. The interpretation of the MODE, ASID, and PPN +fields is the same as for {\tt satp}. + +\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 background supervisor address translation and protection register {\tt satp}.} +\label{rv32bsatpreg} +\end{figure} + +\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 background supervisor address translation and protection register {\tt satp}, for MODE +values Bare, Sv39, and Sv48.} +\label{rv64bsatpreg} +\end{figure*} + +\section{Hypervisor Instructions} + +The hypervisor extension adds one new instruction, HRET, which is valid only +in HS-mode when {\tt mstatus}.TSR=0, or in M-mode (irrespective of {\tt +mstatus}.TSR). HRET sets {\tt hstatus}.SPV=1, then performs an SRET. + +\begin{commentary} +As compared to setting {\tt hstatus}.SPV then executing SRET, HRET halves the +number of emulation traps when executing an HS-mode hypervisor on an +implementation without the hypervisor extension, or when recursively +virtualizing. The need to set {\tt hstatus}.SPV before executing SRET arises +when the hypervisor takes an interrupt while servicing a trap from a guest, +because the nested interrupt clears {\tt hstatus}.SPV. \end{commentary} + +\section{Machine-Level CSRs} + +The hypervisor extension adds one new machine-level CSR, {\tt mvtval}, and +augments the {\tt mstatus} CSR. + +\subsection{Machine Virtual Trap Value ({\tt mvtval}) Register} + +The {\tt mvtval} register is an XLEN-bit read-write register formatted as shown +in Figure~\ref{mvtvalreg}. When an access fault, page fault, or misaligned +address exception is taken into M-mode, {\tt mvtval} is +written with the original virtual address that caused the exception. +For other traps into M-mode, {\tt mvtval} is written with zero. + +\begin{figure}[h!] +{\footnotesize +\begin{center} +\begin{tabular}{@{}J} +\instbitrange{XLEN-1}{0} \\ +\hline +\multicolumn{1}{|c|}{\tt mvtval} \\ +\hline +XLEN \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Machine Virtual Trap Value register ({\tt mvtval}).} +\label{mvtvalreg} +\end{figure} + +\subsection{Machine Status Register ({\tt mstatus})} + +The hypervisor extension adds two fields to the machine-mode {\tt mstatus} CSR, +MPV and MTL, +and modifies the behavior of several existing fields. +Figure~\ref{hypervisor-mstatus} shows the {\tt mstatus} register when the +hypervisor extension is provided. + +\begin{figure*}[h!] +{\footnotesize +\begin{center} +\setlength{\tabcolsep}{4pt} +\begin{tabular}{cYccYccccccc} +\\ +\instbit{XLEN-1} & +\instbitrange{XLEN-2}{36} & +\instbitrange{35}{34} & +\instbitrange{33}{32} & +\instbitrange{31}{23} & +\instbit{22} & +\instbit{21} & +\instbit{20} & +\instbit{19} & +\instbit{18} & +\instbit{17} & + \\ +\hline +\multicolumn{1}{|c|}{SD} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{SXL[1:0]} & +\multicolumn{1}{c|}{UXL[1:0]} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{TSR} & +\multicolumn{1}{c|}{TW} & +\multicolumn{1}{c|}{TVM} & +\multicolumn{1}{c|}{MXR} & +\multicolumn{1}{c|}{SUM} & +\multicolumn{1}{c|}{MPRV} & + \\ +\hline +1 & XLEN-37 & 2 & 2 & 9 & 1 & 1 & 1 & 1 & 1 & 1 & \\ +\end{tabular} +\begin{tabular}{ccccccccccccccc} +\\ +& +\instbitrange{16}{15} & +\instbitrange{14}{13} & +\instbitrange{12}{11} & +\instbit{10} & +\instbit{9} & +\instbit{8} & +\instbit{7} & +\instbit{6} & +\instbit{5} & +\instbit{4} & +\instbit{3} & +\instbit{2} & +\instbit{1} & +\instbit{0} \\ +\hline + & +\multicolumn{1}{|c|}{XS[1:0]} & +\multicolumn{1}{c|}{FS[1:0]} & +\multicolumn{1}{c|}{MPP[1:0]} & +\multicolumn{1}{c|}{MPV} & +\multicolumn{1}{c|}{MTL} & +\multicolumn{1}{c|}{SPP} & +\multicolumn{1}{c|}{MPIE} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{SPIE} & +\multicolumn{1}{c|}{UPIE} & +\multicolumn{1}{c|}{MIE} & +\multicolumn{1}{c|}{\wpri} & +\multicolumn{1}{c|}{SIE} & +\multicolumn{1}{c|}{UIE} \\ +\hline + & 2 & 2 & 2 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \\ +\end{tabular} +\end{center} +} +\vspace{-0.1in} +\caption{Machine-mode status register ({\tt mstatus}) for RV64 and RV128.} +\label{hypervisor-mstatus} +\end{figure*} + +The MPV bit (Machine Previous Virtualization Mode) is written by the implementation +whenever a trap is taken into M-mode. Just as the MPP bit is set to the privilege +mode at the time of the trap, the MPV bit is set to the value of the virtualization +mode V at the time of the trap. When an MRET instruction is executed, the +virtualization mode V is set to MPV, unless MPP=3, in which case V remains 0. + +The MTL bit (Machine Translation Level), which indicates which address-translation level +caused a page-fault exception, is also written by the implementation whenever a trap +is taken into M-mode. On an access fault, or on a page fault due to HS-level address +translation, MTL is set to 0. For any other trap into M-mode, MTL is set to the value +of V at the time of the trap. + +The SXL field controls the value of XLEN for HS-mode. +The UXL field controls the value of XLEN for S-mode when V=1, or for U-mode when V=0. + +The TSR and TVM fields only affect execution in HS-mode. + +The TW field affects execution in both HS-mode and S-mode. + +The hypervisor extension changes the behavior of the the Modify Privilege +field, MPRV. When MPRV=0, translation and protection behave as normal. When +MPRV=1, loads and stores are translated and protected as though the current +privilege mode were set to MPP and the current virtualization mode were set to +MPV. Table~\ref{h-mprv} enumerates the cases. + +\begin{table*}[h!] +\begin{center} +\begin{tabular}{|c|c|c||p{5in}|} + \hline + MPRV & MPV & MPP & Effect \\ \hline \hline + 0 & -- & -- & Normal access; current privilege and virtualization modes apply. \\ \hline + 1 & 0 & 0 & U-level access with HS-level translation and protection only. \\ \hline + 1 & 0 & 1 & HS-level access with HS-level translation and protection only. \\ \hline + 1 & -- & 3 & M-level access with no translation. \\ \hline + 1 & 1 & 0 & U-level access with two-level translation and protection. {\tt sstatus}.MXR makes any executable page readable. {\tt bsstatus}.MXR makes readable those pages marked executable at the S translation level only if readable at the HS translation level. \\ \hline + 1 & 1 & 1 & S-level access with two-level translation and protection. {\tt sstatus}.MXR makes any executable page readable. {\tt bsstatus}.MXR makes readable those pages marked executable at the S translation level only if readable at the HS translation level. {\tt bsstatus}.SUM applies instead of {\tt sstatus}.SUM. \\ \hline + \end{tabular} +\end{center} +\caption{Effect on load and store translation and protection under MPRV. When MPRV=1, MPP$\neq$3, and {\tt hstatus}.SPRV=1, the effective privilege is further modified: {\tt hstatus}.SPV applies instead of MPV, and {\tt sstatus}.SPP applies instead of MPP.} +\label{h-mprv} +\end{table*} + +The {\tt mstatus} register is a superset of the {\tt sstatus} register; +modifying a field in {\tt sstatus} modifies the homonymous field in {\tt +mstatus}, and vice-versa. + +\section{Two-Level Address Translation} + +Whenever the current virtualization mode V is 1, two-level address translation +and protection is in effect. For any virtual memory access, the original +virtual address is first converted +by S-level address translation, as controlled by the {\tt satp} register, into +a {\em guest physical address}. The guest physical address is then +converted by HS-level address translation, as controlled by the {\tt bsatp} +register, into a {\em machine physical address}. +Although there is no option to disable two-level address translation when V=1, +either level of translation can be effectively disabled by zeroing the +corresponding {\tt satp} or {\tt bsatp} register. + +For the purposes of HS-level address translation and protection, all memory +accesses made with V=1---including those made by the S-level +address-translation hardware---are considered user-level accesses. In +addition to satisfying S-level translation and protection, the access type +must be permitted at user level by HS-level translation and protection. +The user page protections at HS level are perceived by S-mode as physical +memory protection. Figure~\ref{hs-pte-perm} summarizes the effective +permissions at each translation level. + +\begin{table*}[h!] +\begin{center} +\begin{tabular}{|c|c||l|l|l|} + \hline + Virtualization & Privilege & Permissions at S & Permissions at HS \\ + Mode (V) & Mode & translation level & translation level \\ \hline + 0 & U & --- & User \\ + 0 & HS & --- & Supervisor \\ + \hline + 1 & U & User & User \\ + 1 & S & Supervisor & User \\ + \hline + \end{tabular} +\end{center} +\caption{Effective virtual-memory permissions at each address-translation level.} +\label{hs-pte-perm} +\end{table*} + +An HS-level memory protection fault caused by an access made by the +S-level address translation hardware raises a load or store page +fault exception. HS-level memory protection faults caused by other +accesses with V = 1 raise the page-fault exception corresponding to +the original access type (instruction, load, or store/AMO). HS-level +memory protection faults are treated as HS-level exceptions for the purpose of +exception delegation, and so are not delegated to S-mode, regardless of the +setting of the {\tt hedeleg} register. + +Note that the S-level MXR setting, which makes execute-only pages readable, +only overrides S-level page protection. Setting MXR at S-level does not override +HS-level page protections. Setting MXR at HS-level, however, overrides +both HS-level and S-level execute-only permissions. + +For the purposes of HS-level address translation protection, memory accesses +made in HS mode are considered supervisor-level accesses. For example, for an +HS-mode virtual memory access to succeed, the corresponding HS-level page-table +entry must not have its U bit set, unless overridden by {\tt sstatus}.SUM. + +Machine-level physical memory protection applies to machine physical +addresses and is in effect regardless of virtualization mode. + +\subsection{Memory-Management Fences} + +The behavior of the SFENCE.VMA instruction is affected by the current +virtualization mode V. When V=0, the virtual-address argument is an HS-level +virtual address, and the ASID argument is an HS-level ASID. If either argument +is provided, the instruction orders stores only to HS-level address-translation +structures with subsequent address translations. If neither argument is +provided, the instruction orders stores to all HS-level and S-level address-translation structures +with subsequent address translations. + +When V=1, the virtual-address argument to SFENCE.VMA is a guest virtual +address, and the ASID argument is an S-level ASID. The instruction +orders stores only to the S-level address-translation structures within the +HS-level address-space specified by the {\tt bsatp} register. + +When V=0, attempts to execute SFENCE.VMA in U-mode or when {\tt mstatus}.TSR=1 +raise an illegal instruction exception. When V=1, attempts to execute +SFENCE.VMA in U-mode or when {\tt hstatus}.VTSR=1 raise an illegal instruction +exception. + +\subsection{Trap Value Register Discipline} + +For an access fault, or for a page fault due to HS-level address translation, +if V=1 at the time of the trap, then {\tt mtval} or {\tt stval} is written +with the host virtual address (i.e., the guest physical address) obtained from +translation of the original virtual address by S-level address translation. +For any other access fault, page fault, or misaligned address exception, {\tt +mtval} or {\tt stval} is written with the original virtual address, as usual. + +When a trap is taken into M-mode that sets {\tt mstatus}.MPV=1 and {\tt +mstatus}.MTL=0, register {\tt mvtval} contains the access's original virtual +address (guest virtual address) and {\tt mtval} contains the host virtual +address (guest physical address) after S-level address translation. Likewise, +when a trap is taken into HS-mode that sets {\tt hstatus}.SPV=1 and {\tt +hstatus}.STL=0, {\tt hvtval} contains the original virtual address (guest +virtual address) and {\tt stval} contains the host virtual address (guest +physical address) after S-level address translation. + +\section{Base ISA Control} + +The {\tt mstatus} field SXL determines XLEN for HS-mode. + +When executing in S-mode, XLEN is determined by the the UXL field of the +background register {\tt bsstatus}. Because {\tt bsstatus} is swapped with +{\tt sstatus} when transitioning from S-mode into HS-mode or M-mode, HS-mode and +M-mode control S-mode's XLEN via the UXL field of the foreground register {\tt +sstatus}. + +When executing in U-mode, XLEN is determined by the UXL field of the foreground register {\tt sstatus}. + +\begin{commentary} +HS-mode controls unvirtualized U-mode's XLEN the same way it controls virtualized S-mode's XLEN, via +{\tt sstatus}.UXL. +\end{commentary} + +\section{Traps} + +The hypervisor extension modifies the environment-call exception cause +encoding. Environment calls from HS-mode use cause 9, whereas environment +calls from an S-mode guest now use cause 10. Table~\ref{hcauses} lists the +possible M-mode and HS-mode exception codes when the hypervisor extension is +present. + +\begin{commentary} +HS-mode and S-mode ECALLs use different cause values so they can be delegated +separately. Without the hypervisor extension, cause 9 is used for S-mode +environment calls. Using cause 9 for HS-mode environment calls when the +hypervisor extension is enabled simplifies M-mode software. +\end{commentary} + +\begin{table*}[h!] +\begin{center} +\begin{tabular}{|r|r|l|l|} + + \hline + Interrupt & Exception Code & Description \\ + \hline + 1 & 0 & User software interrupt \\ + 1 & 1 & Supervisor software interrupt \\ + 1 & 2 & {\em Reserved} \\ + 1 & 3 & Machine software interrupt \\ + 1 & 4 & User timer interrupt \\ + 1 & 5 & Supervisor timer interrupt \\ + 1 & 6 & {\em Reserved} \\ + 1 & 7 & Machine timer interrupt \\ + 1 & 8 & User external interrupt \\ + 1 & 9 & Supervisor external interrupt \\ + 1 & 10 & {\em Reserved} \\ + 1 & 11 & Machine external interrupt \\ + 1 & $\ge$12 & {\em Reserved} \\ \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 HS-mode \\ + 0 & 10 & Environment call from S-mode \\ + 0 & 11 & Environment call from M-mode \\ + 0 & 12 & Instruction page fault \\ + 0 & 13 & Load page fault \\ + 0 & 14 & {\em Reserved} \\ + 0 & 15 & Store/AMO page fault \\ + 0 & $\ge$16 & {\em Reserved} \\ + \hline +\end{tabular} +\end{center} +\caption{Supervisor and machine cause register ({\tt scause} and {\tt mcause}) values when the hypervisor extension is enabled.} +\label{hcauses} +\end{table*} + +When a trap occurs in HS-mode, or in U-mode with V=0, it goes to M-mode, unless +delegated by {\tt medeleg} or {\tt mideleg}, in which case it goes to HS-mode. +If the N extension for user-mode interrupts is implemented, then U-mode (V=0) +traps destined for HS-mode may be further delegated to U-mode using the {\tt +sedeleg} and {\tt sideleg} CSRs. + +When a trap occurs in S-mode, or in U-mode with V=1, it goes to M-mode, unless +delegated by {\tt medeleg} or {\tt mideleg}, in which case it goes to HS-mode, +unless further delegated by {\tt hedeleg} or {\tt hideleg}, in which case it +goes to S-mode. If the N extension for user-mode interrupts is implemented, +then U-mode traps destined for S-mode may be further delegated to U-mode +using the {\tt sedeleg} and {\tt sideleg} CSRs. + +When a trap is taken into M-mode, the following occurs: first, if the +virtualization mode V was 1, the contents of the background supervisor +registers are swapped with their foreground counterparts. Then, {\tt +mstatus}.MPV and {\tt mstatus}.MPP are set according to Table~\ref{h-mpp}. + +\begin{table*}[h!] +\begin{center} +\begin{tabular}{|l|c|c|} + \hline + Previous Mode & MPV & MPP \\ \hline + U-mode, V=0 & 0 & 0 \\ + HS-mode & 0 & 1 \\ + M-mode & 0 & 3 \\ \hline + U-mode, V=1 & 1 & 0 \\ + S-mode & 1 & 1 \\ \hline +\end{tabular} +\end{center} +\caption{Value of {\tt mstatus} fields MPV and MPP after a trap into M-mode. +Upon trap return, MPV is ignored when MPP=3.} +\label{h-mpp} +\end{table*} + +When a trap is taken into HS-mode, the following occurs: first, if the +virtualization mode V was 1, the contents of the background supervisor +registers are swapped with their foreground counterparts. Then, {\tt +hstatus}.SPV and {\tt sstatus}.SPP are set according to Table~\ref{h-spp}. + +\begin{table*}[h!] +\begin{center} +\begin{tabular}{|l|c|c|} + \hline + Previous Mode & SPV & SPP \\ \hline + U-mode, V=0 & 0 & 0 \\ + HS-mode & 0 & 1 \\ \hline + U-mode, V=1 & 1 & 0 \\ + S-mode & 1 & 1 \\ \hline +\end{tabular} +\end{center} +\caption{Value of {\tt hstatus} field SPV and {\tt sstatus} field SPP after a trap into HS-mode.} +\label{h-spp} +\end{table*} + +When a trap is taken into S-mode, {\tt sstatus}.SPP is set according to +Table~\ref{h-vspp}. The {\tt hstatus}.SPV +bit is not modified, and the current virtualization state V remains 1. + +\begin{table*}[h!] +\begin{center} +\begin{tabular}{|l|c|c|} + \hline + Previous Mode & SPP \\ \hline + U-mode, V=1 & 0 \\ + S-mode & 1 \\ \hline +\end{tabular} +\end{center} +\caption{Value of {\tt sstatus} field SPP after a trap into S-mode.} +\label{h-vspp} +\end{table*} + +\section{Trap Return} + +The MRET instruction is used to return from a trap taken into M-mode. MRET sets the +privilege mode according to the values in {\tt mstatus}.MPP and {\tt +mstatus}.MPV, as encoded in Table~\ref{h-mpp}. MRET then sets {\tt pc}={\tt +mepc}, then in {\tt mstatus} sets MPP=0, MIE=MPIE, then MPIE=1. Finally, if +MRET changed the current virtualization state V, the contents of the +background supervisor registers are swapped with their foreground +counterparts. + +The SRET instruction is usually used to return from a trap taken into HS-mode or +S-mode. Its behavior depends on the current virtualization mode. When +executed in M-mode or HS-mode (i.e., V=0), SRET sets the privilege mode +according to the values in {\tt sstatus}.SPP and {\tt hstatus}.SPV, as encoded +in Table~\ref{h-spp}. When executed in S-mode (i.e., V=1), SRET sets the +privilege mode according to Table~\ref{h-vspp}. In either case, SRET then +sets {\tt pc}={\tt sepc}, then in {\tt sstatus} sets SPP=0, SIE=SPIE, then +SPIE=1. Finally, if SRET changed the current virtualization state V, the +contents of the background supervisor registers are swapped with their +foreground counterparts. + +The HRET instruction can be used to return from HS-mode into a virtualized +guest. It first sets {\tt sstatus.SPV}=1, then performs the same actions as +SRET. |