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Re: [PATCH 0/8] RISC V partial support for 128-bit architecture

From: Frédéric Pétrot
Subject: Re: [PATCH 0/8] RISC V partial support for 128-bit architecture
Date: Tue, 31 Aug 2021 19:25:46 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

Le 31/08/2021 à 05:15, Alistair Francis a écrit :
> On Tue, Aug 31, 2021 at 5:29 AM Frédéric Pétrot
> <frederic.petrot@univ-grenoble-alpes.fr> wrote:
>> This series of patches aims at adding partial 128-bit support to the riscv
>> target, following the (unratified) RV128I specification, Chapter 7 of
>> riscv-spec document dated 20191214.
>> It provides support for all user integer (I) instructions and for an M
>> extension which follows the definition of the 32 and 64-bit specifications.
>> We also included minimal support for 128-bit csrs.
>> Among the csrs, we selected misa, to know the mxlen in which the processor
>> is, mtvec, mepc, mscratch and mstatus for minimal kernel development, and
>> satp to point to the page table.
>> We fallback on the 64-bit csrs for the others.
>> In the last patch, we propose a "natural" extension of the sv39 and sv48
>> virtual address modes using 16KB pages, that we believe suitable for
>> 128-bit CPU workloads.
>> There are two strong assumptions in this implementation:
>> - the 64 upper bits of the addresses are irrelevant, be they virtual or
>>   physical, in order to use the existing address translation mechanism,
>> - the mxlen field stays hardwired, so there is no dynamic change in
>>   register size.
>> As no toolchain exists yet for this target, we wrote all our tests in asm
>> using macros expanding .insn directives.
>> We unit tested the behavior of the instructions, and wrote some simple
>> user level performance tests: on our examples the execution speed of the
>> 128-bit version is between 1.2 to 5 time slower than its 32 and 64-bit
>> counterparts.
> Are you able to share these tests? I would like to add them to my
> RISC-V tests so that I can catch any regressions

  I made a repo on github: git@github.com:fpetrot/128-test.git
  Note that the unit tests are nicely wrapped up, while the functional
  ones are more targeting the human, so to say.
  Feel free to use as you wish.


> Alistair

| Frédéric Pétrot, Pr. Grenoble INP-Ensimag/TIMA,   Ensimag deputy director |
| Mob/Pho: +33 6 74 57 99 65/+33 4 76 57 48 70      Ad augusta  per angusta |
| http://tima.univ-grenoble-alpes.fr frederic.petrot@univ-grenoble-alpes.fr |

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