Re: [PATCH] Add an ISA requirement section


atishp@...
 

On Thu, 2021-07-22 at 01:17 -0700, Andrew Waterman wrote:
Thanks for adding this.

For systems that support little-endian, the ISA spec already requires
that mstatus.MBE be reset to 0.  Furthermore, the ISA spec does not
permit hardwiring SBE or UBE to a value that MBE does not support. 
(And of course the reset values of SBE and UBE are immaterial, since
they don't affect M-mode software and M-mode software must initialize
the rest of mstatus prior to entering S-mode or U-mode.)

If we combine all of these facts, we can replace the following
sentence

  Platforms must fully support Little-Endian operation and must come
up in this mode of operation after power-on reset

with the more precise constraint

  Implementations must not hardwire the mstatus.MBE field to 1.
ok. I will update it. Just a suggestion:

Is it better to say what must be supported rather what is not and be
more verbose? Something like this:

Platforms is allowed to operate only in little-endian mode i.e.
implementations must hardwire the mstatus.MBE field to 0.


and we will still obtain the desired result.

On Thu, Jul 22, 2021 at 1:00 AM Atish Patra <atish.patra@...>
wrote:
There are few ISA level requirements/strong recommendations that
platform
should follow. Create an new subsection to contain all these
requirements.
Move the user-level requirements from user.doc to here as well.

Signed-off-by: Atish Patra <atish.patra@...>
---
 riscv-platform-spec.adoc | 11 +++++++++++
 user-level.adoc          | 17 -----------------
 2 files changed, 11 insertions(+), 17 deletions(-)
 delete mode 100644 user-level.adoc

diff --git a/riscv-platform-spec.adoc b/riscv-platform-spec.adoc
index 394304bd6778..1f5b29a0200b 100644
--- a/riscv-platform-spec.adoc
+++ b/riscv-platform-spec.adoc
@@ -115,6 +115,17 @@ The M platform has the following extensions:
 ** 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.
+* ISA Requirements
+** 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
+  values.)
+** Platforms must fully support Little-Endian operation and must
come
+up in this mode of operation after power-on reset.
+** User-mode programs should not execute the `fence.i`
instruction.


 ==== PMU
diff --git a/user-level.adoc b/user-level.adoc
deleted file mode 100644
index 2742d98acaac..000000000000
--- a/user-level.adoc
+++ /dev/null
@@ -1,17 +0,0 @@
-// SPDX-License-Indentifer: CC-BY-4.0
-//
-// user-level.adoc: original User Level Platform content
-//
-// This is material from the very first draft of the spec.
-//
-
-## User-Level Platform
-
-* 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
-  values.)
-
--
Regards,
Atish

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