Messages in this thread | | | Date | Wed, 28 Sep 2022 16:50:15 -0400 | Subject | Re: [PATCH v3 05/28] KVM: x86: Don't inhibit APICv/AVIC if xAPIC ID mismatch is due to 32-bit ID | From | Alejandro Jimenez <> |
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On 9/28/2022 12:51 PM, 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? The patch below fixes the problems for all the scenarios I have tested so far.
Thank you, Alejandro
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". > > 2. The refresh needs to be noted after 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). > > --- > 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
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