From: Sean Christopherson seanjc@google.com
commit f651a008954803d7bb2d85b7042d0fd46133d782 upstream.
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.
Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base") Reported-by: Suravee Suthikulpanit suravee.suthikulpanit@amd.com Cc: stable@vger.kernel.org Reviewed-by: Maxim Levitsky mlevitsk@redhat.com Signed-off-by: Sean Christopherson seanjc@google.com Message-Id: 20230106011306.85230-7-seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kvm/lapic.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
--- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -2078,7 +2078,12 @@ static void kvm_lapic_xapic_id_updated(s 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);