Re: [PATCH v5 1/5] arm64: Filter out SVE hwcaps when FEAT_SVE isn't implemented

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Jan 07, 2025 at 10:59:41PM +0000, Mark Brown wrote:
> From: Marc Zyngier <maz@kernel.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@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: stable@vger.kernel.org

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