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

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Date: Thu, 15 Feb 2024 08:39:26 -0600
From: Peter Bergner <bergner@...ux.ibm.com>
To: 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>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v2] uapi/auxvec: Define AT_HWCAP3 and AT_HWCAP4 aux
 vector, entries

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.

Peter