[PATCH v3] arm64: Filter out SVE hwcaps when FEAT_SVE isn't implemented

[Mark Brown]

On Mon, Jan 06, 2025 at 05:40:20PM +0000, Marc Zyngier wrote:

> 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.

Reviewed-by: Mark Brown <broonie at kernel.org>

This is probably our best option here, it's what we really meant anyway
- SME did retroactively complicate the meaning of the fields so it's not
unreasaonable for userspace to get confused.  For SME specific usage we
should implement separate SME capabilities that have a similar has_sme()
check, I'll look into that.

I agree with the discussion on v2 that anything reading the ID registers
from userspace needs to have a similar understanding that SME only
systems exist so we shouldn't do anything there.
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