KVM: SVM: Document that vCPU ID == APIC ID in AVIC kick fastpatch

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>
Message-Id: <20230106011306.85230-16-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index e4b5f8b..eb2ad5b 100644 (file)
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -368,8 +368,8 @@ 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))
@@ -397,7 +397,7 @@ 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;
 
@@ -406,9 +406,13 @@ 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;