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

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Date:   Wed, 28 Sep 2022 21:16:00 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Alejandro Jimenez <alejandro.j.jimenez@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH v3 05/28] KVM: x86: Don't inhibit APICv/AVIC if xAPIC ID
 mismatch is due to 32-bit ID

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!

Best regards,
	Maxim Levitsky

> 
> 	if (kvm_xapic_id(apic) == apic->vcpu->vcpu_id)
> 		return;
> 
> Calling that before the internal state has been set with the incoming state from
> userspace is simply wrong.
> 
> The check that the x2APIC ID is "correct" stays where it is, this is purely the
> "is the xAPIC ID different" path.
>