linux-kernel - Re: [PATCH v2] uapi/auxvec: Define AT_HWCAP3 and AT

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Message-ID: <87edddp48o.fsf@mail.lhotse>
Date: Fri, 16 Feb 2024 12:49:59 +1100
From: Michael Ellerman <mpe@...erman.id.au>
To: Peter Bergner <bergner@...ux.ibm.com>, Arnd Bergmann <arnd@...nel.org>,
 linux-api@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Cc: Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>, Szabolcs Nagy
 <szabolcs.nagy@....com>, Nick Piggin <npiggin@....ibm.com>
Subject: Re: [PATCH v2] uapi/auxvec: Define AT_HWCAP3 and AT_HWCAP4 aux
 vector, entries

Peter Bergner <bergner@...ux.ibm.com> writes:
> On 2/15/24 2:16 AM, Arnd Bergmann wrote:
>> On Wed, Feb 14, 2024, at 23:34, Peter Bergner wrote:
>>> The powerpc toolchain keeps a copy of the HWCAP bit masks in our TCB for fast
>>> access by the __builtin_cpu_supports built-in function.  The TCB space for
>>> the HWCAP entries - which are created in pairs - is an ABI extension, so
>>> waiting to create the space for HWCAP3 and HWCAP4 until we need them is
>>> problematical.  Define AT_HWCAP3 and AT_HWCAP4 in the generic uapi header
>>> so they can be used in glibc to reserve space in the powerpc TCB for their
>>> future use.
>>>
>>> I scanned through the Linux and GLIBC source codes looking for unused AT_*
>>> values and 29 and 30 did not seem to be used, so they are what I went
>>> with.  This has received Acked-by's from both GLIBC and Linux kernel
>>> developers and no reservations or Nacks from anyone.
>>>
>>> Arnd, we seem to have consensus on the patch below.  Is this something
>>> you could take and apply to your tree? 
>>>
>> 
>> I don't mind taking it, but it may be better to use the
>> powerpc tree if that is where it's actually being used.
>
> So this is not a powerpc only patch, but we may be the first arch
> to use it.  Szabolcs mentioned that aarch64 was pretty quickly filling
> up their AT_HWCAP2 and that they will eventually require using AT_HWCAP3
> as well.  If you still think this should go through the powerpc tree,
> I can check on that.

I'm happy to take it with Arnd's ack.

I trimmed up the commit message a bit, see below.

cheers


Author:     Peter Bergner <bergner@...ux.ibm.com>
AuthorDate: Wed Feb 14 16:34:06 2024 -0600
Commit:     Michael Ellerman <mpe@...erman.id.au>
CommitDate: Fri Feb 16 12:42:59 2024 +1100

    uapi/auxvec: Define AT_HWCAP3 and AT_HWCAP4 aux vector, entries

    The powerpc toolchain keeps a copy of the HWCAP bit masks in the TCB
    for fast access by the __builtin_cpu_supports() built-in function. The
    TCB space for the HWCAP entries - which are created in pairs - is an ABI
    extension, so waiting to create the space for HWCAP3 and HWCAP4 until
    they are needed is problematic. Define AT_HWCAP3 and AT_HWCAP4 in the
    generic uapi header so they can be used in glibc to reserve space in the
    powerpc TCB for their future use.

    I scanned through the Linux and GLIBC source codes looking for unused
    AT_* values and 29 and 30 did not seem to be used, so they are what I
    went with.

    Signed-off-by: Peter Bergner <bergner@...ux.ibm.com>
    Acked-by: Adhemerval Zanella <adhemerval.zanella@...aro.org>
    Acked-by: Nicholas Piggin <npiggin@...il.com>
    Acked-by: Szabolcs Nagy <szabolcs.nagy@....com>
    Acked-by: Arnd Bergmann <arnd@...db.de>
    Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
    Link: https://msgid.link/a406b535-dc55-4856-8ae9-5a063644a1af@linux.ibm.com