priv-preface.adoc

[colophon]
= Preface

This document describes the RISC-V privileged architecture. This
release, version 20240213, 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_ +
_Smrnmi Extension_ +
*Svade Extension* +
*Svnapot Extension* + 
*Svpbmt Extension* +  
*Svinval Extension* + 
*Svadu Extension* +
*Hypervisor ISA*    
|_1.13_ +
_1.13_ +
_0.1_ +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* +
*1.0* 
|_Draft_ +
_Draft_ +
_Draft_ +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified* +
*Ratified*
|===

The following changes have been made since version 1.12, 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 ISA since
version 1.12:

* Transliterated the document from LaTeX into AsciiDoc.
* 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`.
* Incorporated Svade and Svadu extension specifications.
* 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.

[.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.