linux-kernel - Re: [PATCH v5 3/5] arm64/hwcap: Describe 2024 dpISA extensions to userspace

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Message-ID: <0b3777cc-b73b-4843-97cb-5fe3cb1b9774@sirena.org.uk>
Date: Fri, 7 Feb 2025 18:51:52 +0000
From: Mark Brown <broonie@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Shuah Khan <shuah@...nel.org>, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	kvmarm@...ts.linux.dev, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v5 3/5] arm64/hwcap: Describe 2024 dpISA extensions to
 userspace

On Fri, Feb 07, 2025 at 06:37:27PM +0000, Mark Rutland wrote:
> On Tue, Jan 07, 2025 at 10:59:43PM +0000, Mark Brown wrote:
> > The 2024 dpISA introduces a number of architecture features all of which
> > only add new instructions so only require the addition of hwcaps and ID
> > register visibility.

> While working on SME fixes atop v6.14-rc1, I found this patch breaks the
> build spectacularly with CONFIG_ARM64_SME is enabled (splat at the end
> of this mail), due to some missing definitions.

Yes, I already sent a fix for that which Will has merged:

   https://lore.kernel.org/r/20250203-arm64-remove-sf8mmx-v1-1-6f1da3dbff82@kernel.org
 
> From a quick scan, there are some other missing bits too. For example
> these two caps are defined in terms of ID_AA64FPFR0_EL1 fields:

> > +HWCAP_F8MM8
> > +    Functionality implied by ID_AA64FPFR0_EL1.F8MM8 == 0b0001.
> > +
> > +HWCAP_F8MM4
> > +    Functionality implied by ID_AA64FPFR0_EL1.F8MM4 == 0b0001.

> > +#define KERNEL_HWCAP_F8MM8		__khwcap_feature(F8MM8)
> > +#define KERNEL_HWCAP_F8MM4		__khwcap_feature(F8MM4)

> > +#define HWCAP_F8MM8		(1UL << 35)
> > +#define HWCAP_F8MM4		(1UL << 36)

> ... and we expose the ID register bits to userspace:

> ... but there's no corresponding arm64_elf_hwcaps additions to actually
> expose the hwcaps.

Right, indeed.  I'll send a fix for that.

> There are *SME* variants in arm64_elf_hwcaps:

...

> ... but those KERNEL_HWCAP_SME_* values are never defined, and neither
> are the UAPI equivalents.

> We need to fix that quick, in case we need to shuffle values.

These were removed in the patch above, the relevant features were
removed from the architecture between the 2024-09 and 2024-12 XML
releases so were removed from the patch series by the time it got
applied but with CONFIG_SME being disabled the stray reference wasn't
caught by testing.

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