Re: [PATCH 6/6] Update the ISA requirement section

Heinrich Schuchardt

Am 19. November 2021 01:09:34 MEZ schrieb atishp@...:
Some of the ISA requirement sections do not belong to a platform
specification and can move to the profile specification. The fence.i
recommendation belong to software section as it talks about a requirement
for userspace applications. While at it, provide a better clarification
for the above requirement as well. Thanks Andrew & Greg for reviewing
this section.

Update the ISA requirement section to reflect that.

Suggested-by: Greg Favor <gfavor@...>
Signed-off-by: Atish Patra <atishp@...>
riscv-platform-spec.adoc | 38 +++++++++++++-------------------------
1 file changed, 13 insertions(+), 25 deletions(-)

diff --git a/riscv-platform-spec.adoc b/riscv-platform-spec.adoc
index 81ae740154c1..3e901bb4c5e0 100644
--- a/riscv-platform-spec.adoc
+++ b/riscv-platform-spec.adoc
@@ -108,43 +108,22 @@ Supervisor level (S-mode/VS-mode) and M-mode/HS-mode respectively.
// Base feature set for OS-A Platform
=== Base
==== ISA Requirements
-===== ISA Profile Requirements
-* The OS-A platform must comply with the following profiles defined by the
-RISC-V profiles specification <<spec_profiles>>.
-** RVA22U profile for user-mode.
-** RVA22S profile for supervisor-mode.
-*** The I, M, A, C extensions must be supported.
===== General
+* This OS-A platform must comply with the RVA22U and RVA22S ISA profiles as
+defined in the RISC-V ISA Profiles specification [11].
* A non-conforming extension that conflicts with a supported standard extensions
must satisfy at least one of the following:
** It must be disabled by default.
** The supported standard extension must be declared as unsupported in all
feature discovery structures used by software. This option is allowed only
if the standard extension is not required.
-* The LR/SC reservation set size must be at least 16B and at most 128B.
-* Cacheable main memory regions must support instruction fetch, AMOArithmetic,
-RsrvEventual, and PTE reads and writes.
-* All unimplemented or disabled opcodes and CSRs must raise an
-Illegal Instruction exception.
-* Within main-memory regions, aligned instruction fetch must be atomic, up to
-the smaller of ILEN and XLEN bits. In particular, if an aligned 4-byte word
-is stored with the `sw` instruction, then any processor attempts to execute
-that word, the processor either fetches the newly stored word, or some previous
-value stored to that location. (That is, the fetched instruction is not an
-unpredictable value, nor is it a hybrid of the bytes of the old and new
-* All harts must adhere to the RVWMO memory model.
* All hart PMA regions for main memory must be marked as coherent.
* Memory accesses by I/O masters can be coherent or non-coherent with respect
to all hart-related caches.
-[underline]*_Recommendation_* +
-User-mode programs should not execute the `fence.i` instruction.

===== Supervisor mode
* sstatus
** sstatus.UBE must support the same access attribute (read-only or writable)
@@ -540,6 +519,15 @@ software components:
Rationale: The platform specification intends to avoid fragmentation and
promotes interoperability.

+* To order older stores before younger instruction fetches, user-level programs must
+use system-supplied library calls (e.g. GNU libc's `__riscv_flush_icache`, which
+invokes the Linux kernel's corresponding vDSO routine), rather than executing
+the `fence.i` instruction directly.
It seems irrelevant if a library call is made or the syscall is invoked directly.

A libc library may be used by some programs but will be unused by others.

The important thing is that the kernel is used for ordering.

Best regards


+Rationale: The `fence.i` instruction only orders the current hart's instruction
+fetches - which is insufficient even for single-threaded programs since a thread
+may migrate to a different hart.
=== Security
** If M-mode is supported in the platform, all machine mode assets, such as
code and data, shall be protected from all non-machine mode accesses from the

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