linux-kernel - Re: [PATCH RFC 0/2] arm64: Add infrastructure for use of AT

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Message-ID: <b8d2f79b-1ef9-4e30-b3eb-a586fa88c4ba@sirena.org.uk>
Date: Thu, 12 Sep 2024 18:29:36 +0100
From: Mark Brown <broonie@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Eric Biederman <ebiederm@...ssion.com>, Kees Cook <kees@...nel.org>,
	Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Yury Khrustalev <yury.khrustalev@....com>,
	Wilco Dijkstra <wilco.dijkstra@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH RFC 0/2] arm64: Add infrastructure for use of AT_HWCAP3

On Thu, Sep 12, 2024 at 05:45:45PM +0100, Catalin Marinas wrote:
> On Fri, Sep 06, 2024 at 12:05:23AM +0100, Mark Brown wrote:

> > We will at some point need to bite this bullet but we need to decide if
> > it's now or later.  Given that we used the high bits of AT_HWCAP2 first
> > and AT_HWCAP3 is already defined it feels like that might be people's
> > preference, in order to minimise churn in serieses adding new HWCAPs
> > it'd be good to get consensus if that's the case or not.

> Since the arm64 ABI documents that only bits 62 and 63 from AT_HWCAP are
> reserved for glibc, I think we should start using the remaining 30 bits
> of AT_HWCAP first before going for AT_HWCAP3. I'm sure we'll go through
> them quickly enough, so these two patches will have to be merged at some
> point.

That does seem like the easiest path for everyone, assuming that there
hasn't been any usage of the remaining spare bits we weren't aware of.

> We'll need an Ack from the (arm64) glibc people on the GCS patch series
> if we are going for bits 32+ in AT_HWCAP.

Yup, hopefully Yuri or Wilco can confirm prior to reposting.  It does
seem like it'd be good for glibc to add whatever support is needed for
HWCAP3/4 now anyway so that whenever we do burn through the remaining
bits on AT_HWCAP there's less friction as we start using them.

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