Re: [PATCH 01/18] KVM: arm64: Always start with clearing SVE flag on load

[Marc Zyngier <maz@kernel.org>]

On Mon, 30 May 2022 15:41:54 +0100,
Mark Brown <broonie@kernel.org> wrote:
> 
> [1  <text/plain; us-ascii (quoted-printable)>]
> On Sat, May 28, 2022 at 12:38:11PM +0100, Marc Zyngier wrote:
> > 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>
> 
> > 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.