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. Best regards, Maxim Levitsky > the traces: > > qemu-system-x86-10147 [222] ..... 1116.519052: > kvm_apicv_inhibit_changed: set reason=8, inhibits=0x120 > qemu-system-x86-10147 [222] ..... 1116.519063: > kvm_apicv_inhibit_changed: cleared reason=8, inhibits=0x20 > qemu-system-x86-10147 [222] ..... 1117.934222: > kvm_apicv_inhibit_changed: set reason=8, inhibits=0x120 > qemu-system-x86-10147 [222] ..... 1117.934233: > kvm_apicv_inhibit_changed: cleared reason=8, inhibits=0x20 > > 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. > > I did not see this issue when testing a similar host kernel that did not > include this current patchset, but instead applied the earlier: > https://lore.kernel.org/lkml/20220909195442.7660-1-suravee.suthikulpanit@xxxxxxx/ > which inspired this [05/23] patch and the follow up [22/28] in this series. > > I am using QEMU built from v7.1.0 upstream tag, plus the patch at: > https://lore.kernel.org/qemu-devel/20220504131639.13570-1-suravee.suthikulpanit@xxxxxxx/ > > Please feel free to request any other data points that might be relevant > and I'll try to collect them. > > Alejandro > > Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base") > > Reported-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx> > > Cc: Maxim Levitsky <mlevitsk@xxxxxxxxxx> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > > --- > > arch/x86/kvm/lapic.c | 7 ++++++- > > 1 file changed, 6 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > > index adac6ca9b7dc..a02defa3f7b5 100644 > > --- a/arch/x86/kvm/lapic.c > > +++ b/arch/x86/kvm/lapic.c > > @@ -2075,7 +2075,12 @@ static void kvm_lapic_xapic_id_updated(struct kvm_lapic *apic) > > if (KVM_BUG_ON(apic_x2apic_mode(apic), kvm)) > > return; > > > > - if (kvm_xapic_id(apic) == apic->vcpu->vcpu_id) > > + /* > > + * Deliberately truncate the vCPU ID when detecting a modified APIC ID > > + * to avoid false positives if the vCPU ID, i.e. x2APIC ID, is a 32-bit > > + * value. > > + */ > > + if (kvm_xapic_id(apic) == (u8)apic->vcpu->vcpu_id) > > return; > > > > kvm_set_apicv_inhibit(apic->vcpu->kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
