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

Maxim Levitsky
2022-09-28T17:51:50Z

On Wed, 2022-09-28 at 16:51 +0000, Sean Christopherson wrote:
> On Wed, Sep 28, 2022, Maxim Levitsky wrote:
> > On Tue, 2022-09-27 at 23:15 -0400, Alejandro Jimenez wrote:
> > > On 9/20/2022 7:31 PM, Sean Christopherson wrote:
> > > > Truncate the vcpu_id, a.k.a. x2APIC ID, to an 8-bit value when comparing
> > > > it against the xAPIC ID to avoid false positives (sort of) on systems
> > > > with >255 CPUs, i.e. with IDs that don't fit into a u8. The intent of
> > > > APIC_ID_MODIFIED is to inhibit APICv/AVIC when the xAPIC is changed from
> > > > it's original value,
> > > >
> > > > The mismatch isn't technically a false positive, as architecturally the
> > > > xAPIC IDs do end up being aliased in this scenario, and neither APICv
> > > > nor AVIC correctly handles IPI virtualization when there is aliasing.
> > > > However, KVM already deliberately does not honor the aliasing behavior
> > > > that results when an x2APIC ID gets truncated to an xAPIC ID. I.e. the
> > > > resulting APICv/AVIC behavior is aligned with KVM's existing behavior
> > > > when KVM's x2APIC hotplug hack is effectively enabled.
> > > >
> > > > If/when KVM provides a way to disable the hotplug hack, APICv/AVIC can
> > > > piggyback whatever logic disables the optimized APIC map (which is what
> > > > provides the hotplug hack), i.e. so that KVM's optimized map and APIC
> > > > virtualization yield the same behavior.
> > > >
> > > > For now, fix the immediate problem of APIC virtualization being disabled
> > > > for large VMs, which is a much more pressing issue than ensuring KVM
> > > > honors architectural behavior for APIC ID aliasing.
> > >
> > > I built a host kernel with this entire series on top of mainline
> > > v6.0-rc6, and booting a guest with AVIC enabled works as expected on the
> > > initial boot. The issue is that during the first reboot AVIC is
> > > inhibited due to APICV_INHIBIT_REASON_APIC_ID_MODIFIED, and I see
> > > constant inhibition events due to APICV_INHIBIT_REASON_IRQWIN as seen in
> >
> > APICV_INHIBIT_REASON_IRQWIN is OK, because that happens about every time
> > the good old PIT timer fires which happens on reboot.
> >
> > APICV_INHIBIT_REASON_APIC_ID_MODIFIED should not happen as you noted,
> > this needs investigation.
>
> Ya, I'll take a look.
>
> > > 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.

>
> 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));
....

>
> and a third bug in the helper itself, as changes to the ID should be ignored if
> the APIC is hardward disabled since the ID is reset to the vcpu_id when the APIC
> is hardware enabled (architecturally behavior).

That is true, and something I haven't thought about.

Best regards,
Maxim Levitsky

>
> ---
> arch/x86/kvm/lapic.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 804d529d9bfb..b8b2faf5abc7 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2159,6 +2159,9 @@ static void kvm_lapic_xapic_id_updated(struct kvm_lapic *apic)
> {
> struct kvm *kvm = apic->vcpu->kvm;
>
> + if (!kvm_apic_hw_enabled(apic))
> + return;
> +
> if (KVM_BUG_ON(apic_x2apic_mode(apic), kvm))
> return;
>
> @@ -2875,8 +2878,6 @@ static int kvm_apic_state_fixup(struct kvm_vcpu *vcpu,
> icr = __kvm_lapic_get_reg64(s->regs, APIC_ICR);
> __kvm_lapic_set_reg(s->regs, APIC_ICR2, icr >> 32);
> }
> - } else {
> - kvm_lapic_xapic_id_updated(vcpu->arch.apic);
> }
>
> return 0;
> @@ -2912,6 +2913,9 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
> }
> memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
>
> + if (!apic_x2apic_mode(vcpu->arch.apic))
> + kvm_lapic_xapic_id_updated(vcpu->arch.apic);
> +
> atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
> kvm_recalculate_apic_map(vcpu->kvm);
> kvm_apic_set_version(vcpu);
>
> base-commit: 0b502152c0b8523f399bdb53096e2d620c5795b5