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

[Sean Christopherson]

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.