On Mon, 30 May 2022 15:41:54 +0100,
Mark Brown [off-list ref] wrote:

[1  <text/plain; us-ascii (quoted-printable)>]
On Sat, May 28, 2022 at 12:38:11PM +0100, Marc Zyngier wrote:

quoted

On each vcpu load, we set the KVM_ARM64_HOST_SVE_ENABLED
flag if SVE is enabled for EL0 on the host. This is used to restore
the correct state on vpcu put.

However, it appears that nothing ever clears this flag. Once
set, it will stick until the vcpu is destroyed, which has the
potential to spuriously enable SVE for userspace.

Oh dear.

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

quoted

We probably never saw the issue because no VMM uses SVE, but
that's still pretty bad. Unconditionally clearing the flag
on vcpu load addresses the issue.

Unless I'm missing something since we currently always disable
SVE on syscall even if the VMM were using SVE for some reason
(SVE memcpy()?) we should already have disabled SVE for EL0 in
sve_user_discard() during kernel entry so EL0 access to SVE
should be disabled in the system register by the time we get
here.

Indeed. And this begs the question: what is this code actually doing?
Is there any way we can end-up running a guest with any valid host SVE
state?

I remember being >this< close to removing that code some time ago, and
only stopped because I vaguely remembered Dave Martin convincing me at
some point that it was necessary. I'm unable to piece the argument
together again though.

	M.

-- 
Without deviation from the norm, progress is not possible.

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