Re: [PATCH 03/19] Revert "KVM: SVM: Introduce hybrid-AVIC mode"

[Maxim Levitsky]

On Wed, 2022-08-31 at 16:19 +0000, Sean Christopherson wrote:
> On Wed, Aug 31, 2022, Maxim Levitsky wrote:
> > On Wed, 2022-08-31 at 00:34 +0000, Sean Christopherson wrote:
> > > Remove SVM's so called "hybrid-AVIC mode" and reinstate the restriction
> > > where AVIC is disabled if x2APIC is enabled.  The argument that the
> > > "guest is not supposed to access xAPIC mmio when uses x2APIC" is flat out
> > > wrong.  Activating x2APIC completely disables the xAPIC MMIO region,
> > > there is nothing that says the guest must not access that address.
> > > 
> > > Concretely, KVM-Unit-Test's existing "apic" test fails the subtests that
> > > expect accesses to the APIC base region to not be emulated when x2APIC is
> > > enabled.
> > > 
> > > Furthermore, allowing the guest to trigger MMIO emulation in a mode where
> > > KVM doesn't expect such emulation to occur is all kinds of dangerous.
> > > 
> > > Tweak the restriction so that it only inhibits AVIC if x2APIC is actually
> > > enabled instead of inhibiting AVIC is x2APIC is exposed to the guest.
> > > 
> > > This reverts commit 0e311d33bfbef86da130674e8528cc23e6acfe16.
> > 
> > I don't agree with this patch.
> > 
> > When reviewing this code I did note that MMIO is left enabled which is kind
> > of errata on KVM side, and nobody objected to this.
> 
> I didn't object because I didn't read the patch.  I'm very much objecting now.
> 

And I am *very* much objecting to reverting this patch.

Best regards,
	Maxim Levitsky