[PATCH v2 10/23] KVM: SVM: Document that vCPU ID == APIC ID in AVIC kick fastpatch

Sean Christopherson
2022-09-03T00:23:53Z

Document that AVIC is inhibited if any vCPU's APIC ID diverges from its
vCPU ID, i.e. that there's no need to check for a destination match in
the AVIC kick fast path.

Opportunistically tweak comments to remove "guest bug", as that suggests
KVM is punting on error handling, which is not the case. Targeting a
non-existent vCPU or no vCPUs _may_ be a guest software bug, but whether
or not it's a guest bug is irrelevant. Such behavior is architecturally
legal and thus needs to faithfully emulated by KVM (and it is).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/svm/avic.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 8259a64c99d6..b4b5f1422db7 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -395,8 +395,8 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
cluster = (dest >> 4) << 2;
}

+ /* Nothing to do if there are no destinations in the cluster. */
if (unlikely(!bitmap))
- /* guest bug: nobody to send the logical interrupt to */
return 0;

if (!is_power_of_2(bitmap))
@@ -424,7 +424,7 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
if (WARN_ON_ONCE(index != logid_index))
return -EINVAL;

- /* guest bug: non existing/reserved logical destination */
+ /* Nothing to do if the logical destination is invalid. */
if (unlikely(!(logid_entry & AVIC_LOGICAL_ID_ENTRY_VALID_MASK)))
return 0;

@@ -433,9 +433,13 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
}
}

+ /*
+ * KVM inhibits AVIC if any vCPU ID diverges from the vCPUs APIC ID,
+ * i.e. APIC ID == vCPU ID. Once again, nothing to do if the target
+ * vCPU doesn't exist.
+ */
target_vcpu = kvm_get_vcpu_by_id(kvm, l1_physical_id);
if (unlikely(!target_vcpu))
- /* guest bug: non existing vCPU is a target of this IPI*/
return 0;

target_vcpu->arch.apic->irr_pending = true;
--
2.37.2.789.g6183377224-goog