path: root/src/priv-preface.adoc

[colophon]
= Preface

[.big]*_Preface to Version 20241002_*

This document describes the RISC-V privileged architecture. This
release, version 20241002, contains the following versions of the RISC-V ISA
modules:

[%autowidth,float="center",align="center",cols="^,<,^",options="header",]
|===
|Module |Version |Status
|_Machine ISA_ +
*Smstateen Extension* +
*Smcsrind/Sscsrind Extension* +
*Smepmp* +
*Smcntrpmf* +
*Smrnmi Extension* +
*Smcdeleg* +
*Smdbltrp* +
_Supervisor ISA_ +
*Svade Extension* +
*Svnapot Extension* + 
*Svpbmt Extension* +  
*Svinval Extension* + 
*Svadu Extension* +
*Sstc* +
*Sscofpmf* +
*Ssdbltrp* +
*Hypervisor ISA* +
_Shlcofideleg_ +
*Svvptc*

|_1.13_ +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
_1.13_ +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
_1.0_ +
*1.0*

|_Frozen_ +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
_Frozen_ +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
_Frozen_ +
*Ratified*
|===

The following changes have been made since version 1.12 of the Machine and
Supervisor ISAs, which, while not strictly backwards compatible, are not
anticipated to cause software portability problems in practice:

* Redefined `misa`.MXL to be read-only, making MXLEN a constant.
* Added the constraint that SXLEN&#8805;UXLEN.

Additionally, the following compatible changes have been
made to the Machine and Supervisor ISAs since version 1.12:

* Defined the `misa`.B field to reflect that the B extension has been
implemented.
* Defined the `misa`.V field to reflect that the V extension has been
implemented.
* Defined the RV32-only `medelegh` and `hedelegh` CSRs.
* Defined the misaligned atomicity granule PMA, superseding the proposed Zam
  extension.
* Allocated interrupt 13 for Sscofpmf LCOFI interrupt.
* Defined hardware error and software check exception codes.
* Specified synchronization requirements when changing the PBMTE fields
in `menvcfg` and `henvcfg`.
* Exposed count-overflow interrupts to VS-mode via the Shlcofideleg extension.
* Relaxed behavior of some HINTs when MXLEN > XLEN.

Finally, the following clarifications and document improvements have been made
since the last document release:

* Transliterated the document from LaTeX into AsciiDoc.
* Included all ratified extensions through March 2024.
* Clarified that "platform- or custom-use" interrupts are actually
"platform-use interrupts", where the platform can choose to make some custom.
* Clarified semantics of explicit accesses to CSRs wider than XLEN bits.
* Clarified that MXLEN&#8805;SXLEN.
* Clarified that WFI is not a HINT instruction.
* Clarified that VS-stage page-table accesses set G-stage A/D bits.
* Clarified ordering rules when PBMT=IO is used on main-memory regions.
* Clarified ordering rules for hardware A/D bit updates.
* Clarified that, for a given exception cause, `__x__tval` might sometimes
be set to a nonzero value but sometimes not.
* Clarified exception behavior of unimplemented or inaccessible CSRs.
* Clarified that Svpbmt allows implementations to override additional PMAs.
* Replaced the concept of vacant memory regions with inaccessible memory or I/O regions.
* Clarified that timer and count-overflow interrupts' arrival in
  interrupt-pending registers is not immediate.
* Clarified that MXR affects only explicit memory accesses.

[.big]*_Preface to Version 20211203_*

This document describes the RISC-V privileged architecture. This
release, version 20211203, contains the following versions of the RISC-V
ISA modules:

[%autowidth,float="center",align="center",cols="^,<,^",options="header",]
|===
|Module |Version |Status
|*Machine ISA* +
*Supervisor ISA* +   
*Svnapot Extension* +
*Svpbmt Extension* +
*Svinval Extension* +
*Hypervisor ISA* +
|*1.12* +
*1.12* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
|*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified*
|===

The following changes have been made since version 1.11, which, while
not strictly backwards compatible, are not anticipated to cause software
portability problems in practice:

* Changed MRET and SRET to clear `mstatus`.MPRV when leaving M-mode.
* Reserved additional `satp` patterns for future use.
* Stated that the `scause` Exception Code field must implement bits 4–0
at minimum.
* Relaxed I/O regions have been specified to follow RVWMO. The previous
specification implied that PPO rules other than fences and
acquire/release annotations did not apply.
* Constrained the LR/SC reservation set size and shape when using
page-based virtual memory.
* PMP changes require an SFENCE.VMA on any hart that implements
page-based virtual memory, even if VM is not currently enabled.
* Allowed for speculative updates of page table entry A bits.
* Clarify that if the address-translation algorithm non-speculatively
reaches a PTE in which a bit reserved for future standard use is set, a
page-fault exception must be raised.

Additionally, the following compatible changes have been made since
version 1.11:

* Removed the N extension.
* Defined the mandatory RV32-only CSR `mstatush`, which contains most of
the same fields as the upper 32 bits of RV64’s `mstatus`.
* Defined the mandatory CSR `mconfigptr`, which if nonzero contains the
address of a configuration data structure.
* Defined optional `mseccfg` and `mseccfgh` CSRs, which control the
machine’s security configuration.
* Defined `menvcfg`, `henvcfg`, and `senvcfg` CSRs (and RV32-only
`menvcfgh` and `henvcfgh` CSRs), which control various characteristics
of the execution environment.
* Designated part of SYSTEM major opcode for custom use.
* Permitted the unconditional delegation of less-privileged interrupts.
* Added optional big-endian and bi-endian support.
* Made priority of load/store/AMO address-misaligned exceptions
implementation-defined relative to load/store/AMO page-fault and
access-fault exceptions.
* PMP reset values are now platform-defined.
* An additional 48 optional PMP registers have been defined.
* Slightly relaxed the atomicity requirement for A and D bit updates
performed by the implementation.
* Clarify the architectural behavior of address-translation caches
* Added Sv57 and Sv57x4 address translation modes.
* Software breakpoint exceptions are permitted to write either 0 or the
`pc` to `__x__tval`.
* Clarified that bare S-mode need not support the SFENCE.VMA
instruction.
* Specified relaxed constraints for implicit reads of non-idempotent
regions.
* Added the Svnapot Standard Extension, along with the N bit in Sv39,
Sv48, and Sv57 PTEs.
* Added the Svpbmt Standard Extension, along with the PBMT bits in Sv39,
Sv48, and Sv57 PTEs.
* Added the Svinval Standard Extension and associated instructions.

Finally, the hypervisor architecture proposal has been extensively
revised.

[.big]*_Preface to Version 1.11_*

This is version 1.11 of the RISC-V privileged architecture. The document
contains the following versions of the RISC-V ISA modules:

[%autowidth,float="center",align="center",cols="^,<,^",options="header",]
|===
|Module |Version |Status
|*Machine ISA* +
*Supervisor ISA* +
_Hypervisor ISA_ 
|*1.11* +
*1.11* + 
_0.3_ 
|*Ratified* +
*Ratified* +
_Draft_
|===

Changes from version 1.10 include:

* Moved Machine and Supervisor spec to *Ratified* status.
* Improvements to the description and commentary.
* Added a draft proposal for a hypervisor extension.
* Specified which interrupt sources are reserved for standard use.
* Allocated some synchronous exception causes for custom use.
* Specified the priority ordering of synchronous exceptions.
* Added specification that xRET instructions may, but are not required
to, clear LR reservations if A extension present.
* The virtual-memory system no longer permits supervisor mode to execute
instructions from user pages, regardless of the SUM setting.
* Clarified that ASIDs are private to a hart, and added commentary about
the possibility of a future global-ASID extension.
* SFENCE.VMA semantics have been clarified.
* Made the `mstatus`.MPP field *WARL*, rather than *WLRL*.
* Made the unused `__x__ip` fields *WPRI*, rather than *WIRI*.
* Made the unused `misa` fields *WARL*, rather than *WIRI*.
* Made the unused `pmpaddr` and `pmpcfg` fields *WARL*, rather than *WIRI*.
* Required all harts in a system to employ the same PTE-update scheme as
each other.
* Rectified an editing error that misdescribed the mechanism by which
`mstatus.__x__IE` is written upon an exception.
* Described scheme for emulating misaligned AMOs.
* Specified the behavior of the `misa` and `__x__epc` registers in systems
with variable IALIGN.
* Specified the behavior of writing self-contradictory values to the
`misa` register.
* Defined the `mcountinhibit` CSR, which stops performance counters from
incrementing to reduce energy consumption.
* Specified semantics for PMP regions coarser than four bytes.
* Specified contents of CSRs across XLEN modification.
* Moved PLIC chapter into its own document.

[.big]*_Preface to Version 1.10_*

This is version 1.10 of the RISC-V privileged architecture proposal.
Changes from version 1.9.1 include:

* The previous version of this document was released under a Creative
Commons Attribution 4.0 International License by the original authors,
and this and future versions of this document will be released under the
same license.
* The explicit convention on shadow CSR addresses has been removed to
reclaim CSR space. Shadow CSRs can still be added as needed.
* The `mvendorid` register now contains the JEDEC code of the core
provider as opposed to a code supplied by the Foundation. This avoids
redundancy and offloads work from the Foundation.
* The interrupt-enable stack discipline has been simplified.
* An optional mechanism to change the base ISA used by supervisor and
user modes has been added to the `mstatus` CSR, and the field previously
called Base in `misa` has been renamed to `MXL` for consistency.
* Clarified expected use of XS to summarize additional extension state
status fields in `mstatus`.
* Optional vectored interrupt support has been added to the `mtvec` and
`stvec` CSRs.
* The SEIP and UEIP bits in the `mip` CSR have been redefined to support
software injection of external interrupts.
* The `mbadaddr` register has been subsumed by a more general `mtval`
register that can now capture bad instruction bits on an illegal
instruction fault to speed instruction emulation.
* The machine-mode base-and-bounds translation and protection schemes
have been removed from the specification as part of moving the virtual
memory configuration to `sptbr` (now `satp`). Some of the motivation for
the base and bound schemes are now covered by the PMP registers, but
space remains available in `mstatus` to add these back at a later date
if deemed useful.
* In systems with only M-mode, or with both M-mode and U-mode but
without U-mode trap support, the `medeleg` and `mideleg` registers now
do not exist, whereas previously they returned zero.
* Virtual-memory page faults now have `mcause` values distinct from
physical-memory access faults. Page-fault exceptions can now be
delegated to S-mode without delegating exceptions generated by PMA and
PMP checks.
* An optional physical-memory protection (PMP) scheme has been proposed.
* The supervisor virtual memory configuration has been moved from the
`mstatus` register to the `sptbr` register. Accordingly, the `sptbr`
register has been renamed to `satp` (Supervisor Address Translation and
Protection) to reflect its broadened role.
* The SFENCE.VM instruction has been removed in favor of the improved
SFENCE.VMA instruction.
* The `mstatus` bit MXR has been exposed to S-mode via `sstatus`.
* The polarity of the PUM bit in `sstatus` has been inverted to shorten
code sequences involving MXR. The bit has been renamed to SUM.
* Hardware management of page-table entry Accessed and Dirty bits has
been made optional; simpler implementations may trap to software to set
them.
* The counter-enable scheme has changed, so that S-mode can control
availability of counters to U-mode.
* H-mode has been removed, as we are focusing on recursive
virtualization support in S-mode. The encoding space has been reserved
and may be repurposed at a later date.
* A mechanism to improve virtualization performance by trapping S-mode
virtual-memory management operations has been added.
* The Supervisor Binary Interface (SBI) chapter has been removed, so
that it can be maintained as a separate specification.

[.big]*_Preface to Version 1.9.1_*

This is version 1.9.1 of the RISC-V privileged architecture proposal.
Changes from version 1.9 include:

* Numerous additions and improvements to the commentary sections.
* Change configuration string proposal to be use a search process that
supports various formats including Device Tree String and flattened
Device Tree.
* Made `misa` optionally writable to support modifying base and
supported ISA extensions. CSR address of `misa` changed.
* Added description of debug mode and debug CSRs.
* Added a hardware performance monitoring scheme. Simplified the
handling of existing hardware counters, removing privileged versions of
the counters and the corresponding delta registers.
* Fixed description of SPIE in presence of user-level interrupts.