linux-kernel - Re: [PATCH v3 2/3] RISC-V: uapi: add HWCAP for Bitmanip/Scalar Crypto

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Message-ID: <27d0de3e-1006-dd3a-0e91-ae8025ef8426@kernel.org>
Date:   Thu, 24 Nov 2022 17:54:00 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Samuel Ortiz <sameo@...osinc.com>
Cc:     "Hongren (Zenithal) Zheng" <i@...ithal.me>,
        Palmer Dabbelt <palmer@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <atishp@...osinc.com>,
        Anup Patel <anup@...infault.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-man@...r.kernel.org, Jiatai He <jiatai2021@...as.ac.cn>,
        Heiko Stuebner <heiko@...ech.de>
Subject: Re: [PATCH v3 2/3] RISC-V: uapi: add HWCAP for Bitmanip/Scalar Crypto

On 24/11/2022 17:34, Samuel Ortiz wrote:
> On Thu, Nov 24, 2022 at 05:20:37PM +0000, Conor Dooley wrote:
>> On 24/11/2022 17:12, Samuel Ortiz wrote:
>>> [You don't often get email from sameo@...osinc.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
>>>
>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>
>>> On Thu, Nov 24, 2022 at 11:55:01AM +0000, Conor Dooley wrote:
>>>> On Thu, Nov 24, 2022 at 11:47:30AM +0100, Samuel Ortiz wrote:
>>>>
>>>>> Patch #1 is definitely needed regardless of which interface we pick for
>>>>> exposing the ISA strings to userspace.
>>>>
>>>> I took another look at #1, and I feel more confused about what
>>>> constitutes canonical order than I did before! If you know better than
>>>> I, and you probably do since you're interested in these 6 month old
>>>> patches, some insight would be appreciated!
>>>
>>> Assuming we don't go with hwcap, I dont think the order of the
>>> riscv_isa_ext_id enum matters that much?
>>
>> The chief put it in canonical order so that's good enough for me!
>>
>>>
>>> iiuc we're building the cpuinfo string from the riscv_isa_ext_data
>>> array, and I think the current code is incorrect:
>>>
>>> static struct riscv_isa_ext_data isa_ext_arr[] = {
>>>     __RISCV_ISA_EXT_DATA(sscofpmf, RISCV_ISA_EXT_SSCOFPMF),
>>>     __RISCV_ISA_EXT_DATA(sstc, RISCV_ISA_EXT_SSTC),
>>>     __RISCV_ISA_EXT_DATA(svinval, RISCV_ISA_EXT_SVINVAL),
>>>     __RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
>>>     __RISCV_ISA_EXT_DATA(zicbom, RISCV_ISA_EXT_ZICBOM),
>>>     __RISCV_ISA_EXT_DATA(zihintpause, RISCV_ISA_EXT_ZIHINTPAUSE),
>>>     __RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
>>> };
>>>
>>> zicbom and zihintpause should come before supervisor level extensions.
>>> I'm going to send a patch for that.
>>
>> idk, Palmer explicitly re-ordered this:
>> https://lore.kernel.org/linux-riscv/20220920204518.10988-1-palmer@rivosinc.com/
>>
>> By my reading of the isa manual, what Palmer did is correct as
>> those are not "Additional Standard Extensions". /shrug
> 
> Hmm, by their name (Z[a-b]+) they are Additional Standard Extensions.
> What am I missing?

Right, and this is where I get confused. Zam and Ztso *are* Additional
Standard Extensions, I think we can agree on that one? For those
extensions:
\chapter{``Ztso'' Standard Extension for Total Store Ordering, v0.1}
\chapter{``Zam'' Standard Extension for Misaligned Atomics, v0.1}

They're also called out specifically in the table:
https://github.com/riscv/riscv-isa-manual/blob/master/src/naming.tex#L147

For Zihintpause however:
\chapter{``Zihintpause'' Pause Hint, Version 2.0}

See what I mean? I looked at the specs for the bitmanip stuff and for
crypto, which both never mention being standard.

That table has the caption:
> The table also defines the canonical order in which extension names
> must appear in the name string, with top-to-bottom in table
> indicating first-to-last in the name string. 

It only calls out Zicsr, Zifencei, Zam and Ztso are being permitted
before Sdef, but as I said I am not a specs person, so perhaps some
of the extensions in question are intended to go there but have not
yet been merged into the isa manual doc. Zihintpause *is* in the
isa manual though but not specifically called out.

Anyways, hopefully that at least helps with my line of thinking!

Conor.