[PATCH v3 11/26] KVM: SVM: Don't bother checking for "running" AVIC when kicking for IPIs

Sean Christopherson
2021-12-08T01:55:23Z

Drop the avic_vcpu_is_running() check when waking vCPUs in response to a
VM-Exit due to incomplete IPI delivery. The check isn't wrong per se, but
it's not 100% accurate in the sense that it doesn't guarantee that the vCPU
was one of the vCPUs that didn't receive the IPI.

The check isn't required for correctness as blocking == !running in this
context.

From a performance perspective, waking a live task is not expensive as the
only moderately costly operation is a locked operation to temporarily
disable preemption. And if that is indeed a performance issue,
kvm_vcpu_is_blocking() would be a better check than poking into the AVIC.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/svm/avic.c | 15 +++++++++------
arch/x86/kvm/svm/svm.h | 11 -----------
2 files changed, 9 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index e2e4e9f04458..37575b71cdf3 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -295,13 +295,16 @@ static void avic_kick_target_vcpus(struct kvm *kvm, struct kvm_lapic *source,
struct kvm_vcpu *vcpu;
unsigned long i;

+ /*
+ * Wake any target vCPUs that are blocking, i.e. waiting for a wake
+ * event. There's no need to signal doorbells, as hardware has handled
+ * vCPUs that were in guest at the time of the IPI, and vCPUs that have
+ * since entered the guest will have processed pending IRQs at VMRUN.
+ */
kvm_for_each_vcpu(i, vcpu, kvm) {
- bool m = kvm_apic_match_dest(vcpu, source,
- icrl & APIC_SHORT_MASK,
- GET_APIC_DEST_FIELD(icrh),
- icrl & APIC_DEST_MASK);
-
- if (m && !avic_vcpu_is_running(vcpu))
+ if (kvm_apic_match_dest(vcpu, source, icrl & APIC_SHORT_MASK,
+ GET_APIC_DEST_FIELD(icrh),
+ icrl & APIC_DEST_MASK))
kvm_vcpu_wake_up(vcpu);
}
}
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 9f153c59f2c8..ca51d6dfc8e6 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -574,17 +574,6 @@ extern struct kvm_x86_nested_ops svm_nested_ops;

#define VMCB_AVIC_APIC_BAR_MASK 0xFFFFFFFFFF000ULL

-static inline bool avic_vcpu_is_running(struct kvm_vcpu *vcpu)
-{
- struct vcpu_svm *svm = to_svm(vcpu);
- u64 *entry = svm->avic_physical_id_cache;
-
- if (!entry)
- return false;
-
- return (READ_ONCE(*entry) & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK);
-}
-
int avic_ga_log_notifier(u32 ga_tag);
void avic_vm_destroy(struct kvm *kvm);
int avic_vm_init(struct kvm *kvm);
--
2.34.1.400.ga245620fadb-goog