Re: [PATCH v3 05/28] KVM: x86: Don't inhibit APICv/AVIC if xAPIC ID mismatch is due to 32-bit ID

[Sean Christopherson <seanjc@google.com>]

On Wed, Sep 28, 2022, Maxim Levitsky wrote:
> On Wed, 2022-09-28 at 18:03 +0000, Sean Christopherson wrote:
> > On Wed, Sep 28, 2022, Maxim Levitsky wrote:
> > > On Wed, 2022-09-28 at 16:51 +0000, Sean Christopherson wrote:
> > > > > > It happens regardless of vCPU count (tested with 2, 32, 255, 380, and 
> > > > > > 512 vCPUs). This state persists for all subsequent reboots, until the VM 
> > > > > > is terminated. For vCPU counts < 256, when x2apic is disabled the 
> > > > > > problem does not occur, and AVIC continues to work properly after reboots.
> > > > 
> > > > Bit of a shot in the dark, but does the below fix the issue?  There are two
> > > > issues with calling kvm_lapic_xapic_id_updated() from kvm_apic_state_fixup():
> > > > 
> > > >   1. The xAPIC ID should only be refreshed on "set".
> > > True - I didn't bother to fix it yet because it shouldn't cause harm, but
> > > sure this needs to be fixed.
> > 
> > It's probably benign on its own, but with the missing "hardware enabled" check,
> > it could be problematic if userspace does KVM_GET_LAPIC while the APIC is hardware
> > disabled, after the APIC was previously in x2APIC mode.  I'm guessing QEMU does
> > KVM_GET_LAPIC state when emulating reboot, hence the potential for being involved
> > in the bug Alejandro is seeing.
> > 
> > > >   2. The refresh needs to be noted after memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
> > > Are you sure? The check is first because if it fails, then error is returned to userspace
> > > and the KVM's state left unchanged.
> > > 
> > > I assume you are talking about 
> > > 
> > >         ....
> > > 	r = kvm_apic_state_fixup(vcpu, s, true);
> > > 	if (r) {
> > > 		kvm_recalculate_apic_map(vcpu->kvm);
> > > 		return r;
> > > 	}
> > > 	memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
> > 
> > This isn't a failure path though, it's purely a "take note of the update", and
> > KVM needs to do that processing _after_ the actual update.  Specifically,
> > kvm_lapic_xapic_id_updated() consumes the internal APIC state:
> 
> Yes, I somehow blindly assumed that kvm_apic_state_fixup actually checks
> the new state and not the existing state.
> 
> Probably because my original code did that, I think it just checked the 'id'
> variable.. Oh well.
> Thanks for catching this bug!

This was indeed the bug Alejandro hit.  QEMU does KVM_SET_LAPIC and KVM checks
the stale vAPIC state.  Because the vCPU was in x2APIC mode, the 32-bit ID is
in bits 31:0, not bit 31:24, and so kvm_xapic_id() returned '0' instead of the
correct ID.