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". 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 --