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. >