linux-kernel - Re: [PATCH v5 1/5] arm64: Filter out SVE hwcaps when FEAT

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Message-ID: <Z36Abq8mrhnl3cg2@arm.com>
Date: Wed, 8 Jan 2025 13:41:02 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Mark Brown <broonie@...nel.org>
Cc: 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,
	Mark Rutland <mark.rutland@....com>, stable@...r.kernel.org
Subject: Re: [PATCH v5 1/5] arm64: Filter out SVE hwcaps when FEAT_SVE isn't
 implemented

On Tue, Jan 07, 2025 at 10:59:41PM +0000, Mark Brown wrote:
> From: Marc Zyngier <maz@...nel.org>
> 
> The hwcaps code that exposes SVE features to userspace only
> considers ID_AA64ZFR0_EL1, while this is only valid when
> ID_AA64PFR0_EL1.SVE advertises that SVE is actually supported.
> 
> The expectations are that when ID_AA64PFR0_EL1.SVE is 0, the
> ID_AA64ZFR0_EL1 register is also 0. So far, so good.
> 
> Things become a bit more interesting if the HW implements SME.
> In this case, a few ID_AA64ZFR0_EL1 fields indicate *SME*
> features. And these fields overlap with their SVE interpretations.
> But the architecture says that the SME and SVE feature sets must
> match, so we're still hunky-dory.
> 
> This goes wrong if the HW implements SME, but not SVE. In this
> case, we end-up advertising some SVE features to userspace, even
> if the HW has none. That's because we never consider whether SVE
> is actually implemented. Oh well.
> 
> Fix it by restricting all SVE capabilities to ID_AA64PFR0_EL1.SVE
> being non-zero. The HWCAPS documentation is amended to reflect the
> actually checks performed by the kernel.
> 
> Fixes: 06a916feca2b ("arm64: Expose SVE2 features for userspace")
> Reported-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> Signed-off-by: Mark Brown <broonie@...nel.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: stable@...r.kernel.org

Reviewed-by: Catalin Marinas <catalin.marinas@....com>