Re: [PATCH v13 16/40] KVM: arm64: Manage GCS access and registers for guests

[Mark Brown]

On Wed, Oct 02, 2024 at 04:55:25PM +0100, Marc Zyngier wrote:
> Marc Zyngier <maz@xxxxxxxxxx> wrote:

> > > +	if (!kvm_has_gcs(kvm))
> > > +		kvm->arch.fgu[HFGxTR_GROUP] |= (HFGxTR_EL2_nGCS_EL0 |
> > > +						HFGxTR_EL2_nGCS_EL1);

> > Why are you still allowing the GCS instructions when GCS isn't
> > enabled?

> Scratch that, they are NOPs when GCS isn't enabled, so there shouldn't
> be any need for extra traps.

They are, though really they should UNDEF if GCS isn't there (which I
had thought was what you were referencing here).  Equally we only have
traps for a subset of GCS instructions and it's not like there aren't a
whole bunch of untrappable extensions anyway so it's not clear it's
worth the effort just for that.

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