Date:   Wed, 15 Feb 2023 22:43:40 +0000
From:   Jessica Clarke <jrtc27@...c27.com>
To:     Evan Green <evan@...osinc.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Heiko Stübner <heiko@...ech.de>,
        linux-doc@...r.kernel.org,
        Andrew Bresticker <abrestic@...osinc.com>,
        Atish Patra <atishp@...osinc.com>,
        Palmer Dabbelt <palmer@...osinc.com>,
        "Conor.Dooley" <conor.dooley@...rochip.com>,
        Celeste Liu <coelacanthus@...look.com>, slewis@...osinc.com,
        Bagas Sanjaya <bagasdotme@...il.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Jonathan Corbet <corbet@....net>,
        Tobias Klauser <tklauser@...tanz.ch>,
        Andrew Jones <ajones@...tanamicro.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Vineet Gupta <vineetg@...osinc.com>,
        Dao Lu <daolu@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Ruizhe Pan <c141028@...il.com>,
        Anup Patel <apatel@...tanamicro.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Conor Dooley <conor@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>, guoren <guoren@...nel.org>
Subject: Re: [PATCH v2 2/6] RISC-V: Add a syscall for HW probing

On 15 Feb 2023, at 21:14, Evan Green <evan@...osinc.com> wrote:
> 
> On Wed, Feb 15, 2023 at 1:57 AM Arnd Bergmann <arnd@...db.de> wrote:
>> 
>> On Mon, Feb 6, 2023, at 21:14, Evan Green wrote:
>>> We don't have enough space for these all in ELF_HWCAP{,2} and there's no
>>> system call that quite does this, so let's just provide an arch-specific
>>> one to probe for hardware capabilities.  This currently just provides
>>> m{arch,imp,vendor}id, but with the key-value pairs we can pass more in
>>> the future.
>>> 
>>> Co-developed-by: Palmer Dabbelt <palmer@...osinc.com>
>>> Signed-off-by: Palmer Dabbelt <palmer@...osinc.com>
>>> Signed-off-by: Evan Green <evan@...osinc.com>
>> 
>> I'm not sure I understand the problem with
>> AT_HWCAP. While the bits in AT_HWCAP and AT_HWCAP2
>> are limited, I don't see us running out of new
>> AT_* words to use for additional bits. Presumably
>> the kernel would already have to know about the
>> name of each supported HW feature and could assign
>> a unique bit number to them.
> 
> Palmer can probably speak to this with more authority, but my
> understanding about the motivation for an approach like this goes
> something like:
> * With the nature of RISC-V, we expect a lot of these types of bits
> and bobs, many more than we've seen with the likes of x86 and ARM.

We’re already at (I think) 51 standard user-level extensions that LLVM
knows about.

> * We also expect in some cases these values to be inconsistent across CPUs.

That’s also true of some Arm SoCs.

> * While we could copy all that data into the aux vector every time,
> it starts to look like a lot of data, not all programs care about all
> of it, and a lot of it is static, making all the copying wasteful.

Bitvectors are pretty cheap, this is negligible.

> * Another option that would solve most of this would be to point to a
> vDSO data area from the aux vector. This solves the copy complaints,
> but makes that vDSO data ABI, and requires it all to be known up
> front.

That doesn't seem like a huge deal, other than my usual point of
needing a standardised portable cross-platform API for this, so that
shouldn’t be “the” generic interface programmed against by applications.

> * So, a syscall with a vDSO function in front of it seemed like a
> good combination of speed and flexibility.
> 
> You're certainly right that HWCAPn would work for what we're exposing
> today, so the question probably comes down to our relative predictions
> of how this data will grow.

The other big problem is vendor extensions.

Jess

> -Evan
> 
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