If svm_deliver_avic_intr is called just after the target vcpu's AVIC got
inhibited, it might read a stale value of vcpu->arch.apicv_active
which can lead to the target vCPU not noticing the interrupt.
To fix this use load-acquire/store-release so that, if the target vCPU
is IN_GUEST_MODE, we're guaranteed to see a previous disabling of the
AVIC. If AVIC has been disabled in the meanwhile, proceed with the
KVM_REQ_EVENT-based delivery.
All this complicated logic is actually exactly how we can handle an
incomplete IPI vmexit; the only difference lies in who sets IRR, whether
KVM or the processor.
Also incomplete IPI vmexit, has the same races as svm_deliver_avic_intr.
therefore just reuse the avic_kick_target_vcpu for it as well.
Reported-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Co-developed-with: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
---
arch/x86/kvm/svm/avic.c | 85 +++++++++++++++++++++++++----------------
arch/x86/kvm/x86.c | 4 +-
2 files changed, 55 insertions(+), 34 deletions(-)
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 90364d02f22aa..34f62da2fbadd 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -289,6 +289,47 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
return 0;
}
+static void avic_kick_target_vcpu(struct kvm_vcpu *vcpu)
+{
+ bool in_guest_mode;
+
+ /*
+ * vcpu->arch.apicv_active is read after vcpu->mode. Pairs
+ * with smp_store_release in vcpu_enter_guest.
+ */
+ in_guest_mode = (smp_load_acquire(&vcpu->mode) == IN_GUEST_MODE);
+ if (READ_ONCE(vcpu->arch.apicv_active)) {
+ if (in_guest_mode) {
+ /*
+ * Signal the doorbell to tell hardware to inject the IRQ if the vCPU
+ * is in the guest. If the vCPU is not in the guest, hardware will
+ * automatically process AVIC interrupts at VMRUN.
+ *
+ * Note, the vCPU could get migrated to a different pCPU at any
+ * point, which could result in signalling the wrong/previous
+ * pCPU. But if that happens the vCPU is guaranteed to do a
+ * VMRUN (after being migrated) and thus will process pending
+ * interrupts, i.e. a doorbell is not needed (and the spurious
+ * one is harmless).
+ */
+ int cpu = READ_ONCE(vcpu->cpu);
+ if (cpu != get_cpu())
+ wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu));
+ put_cpu();
+ } else {
+ /*
+ * Wake the vCPU if it was blocking. KVM will then detect the
+ * pending IRQ when checking if the vCPU has a wake event.
+ */
+ kvm_vcpu_wake_up(vcpu);
+ }
+ } else {
+ /* Compare this case with __apic_accept_irq. */
+ kvm_make_request(KVM_REQ_EVENT, vcpu);
+ kvm_vcpu_kick(vcpu);
+ }
+}
+
static void avic_kick_target_vcpus(struct kvm *kvm, struct kvm_lapic *source,
u32 icrl, u32 icrh)
{
@@ -304,8 +345,10 @@ static void avic_kick_target_vcpus(struct kvm *kvm, struct kvm_lapic *source,
kvm_for_each_vcpu(i, vcpu, kvm) {
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);
+ icrl & APIC_DEST_MASK)) {
+ vcpu->arch.apic->irr_pending = true;
+ avic_kick_target_vcpu(vcpu);
+ }
}
}
@@ -671,9 +714,12 @@ void svm_load_eoi_exitmap(struct kvm_vcpu *vcpu, u64 *eoi_exit_bitmap)
int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
{
- if (!vcpu->arch.apicv_active)
- return -1;
-
+ /*
+ * Below, we have to handle anyway the case of AVIC being disabled
+ * in the middle of this function, and there is hardly any overhead
+ * if AVIC is disabled. So, we do not bother returning -1 and handle
+ * the kick ourselves for disabled APICv.
+ */
kvm_lapic_set_irr(vec, vcpu->arch.apic);
/*
@@ -684,34 +730,7 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
* the doorbell if the vCPU is already running in the guest.
*/
smp_mb__after_atomic();
-
- /*
- * Signal the doorbell to tell hardware to inject the IRQ if the vCPU
- * is in the guest. If the vCPU is not in the guest, hardware will
- * automatically process AVIC interrupts at VMRUN.
- */
- if (vcpu->mode == IN_GUEST_MODE) {
- int cpu = READ_ONCE(vcpu->cpu);
-
- /*
- * Note, the vCPU could get migrated to a different pCPU at any
- * point, which could result in signalling the wrong/previous
- * pCPU. But if that happens the vCPU is guaranteed to do a
- * VMRUN (after being migrated) and thus will process pending
- * interrupts, i.e. a doorbell is not needed (and the spurious
- * one is harmless).
- */
- if (cpu != get_cpu())
- wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu));
- put_cpu();
- } else {
- /*
- * Wake the vCPU if it was blocking. KVM will then detect the
- * pending IRQ when checking if the vCPU has a wake event.
- */
- kvm_vcpu_wake_up(vcpu);
- }
-
+ avic_kick_target_vcpu(vcpu);
return 0;
}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 85127b3e3690b..81a74d86ee5eb 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9869,7 +9869,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
* result in virtual interrupt delivery.
*/
local_irq_disable();
- vcpu->mode = IN_GUEST_MODE;
+
+ /* Store vcpu->apicv_active before vcpu->mode. */
+ smp_store_release(&vcpu->mode, IN_GUEST_MODE);
srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
--
2.26.3
