[PATCH] KVM: SVM: unconditionally wake up VCPU on IOMMU interrupt

[PATCH] KVM: SVM: unconditionally wake up VCPU on IOMMU interrupt

[Paolo Bonzini]

Checking the mode is unnecessary, and is done without a memory barrier
separating the LAPIC write from the vcpu->mode read; in addition,
kvm_vcpu_wake_up is already doing a check for waiters on the wait queue
that has the same effect.

In practice it's safe because spin_lock has full-barrier semantics on x86,
but don't be too clever.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/svm.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 01e6e8fab5d6..712406f725a2 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1034,15 +1034,12 @@ static int avic_ga_log_notifier(u32 ga_tag)
 	}
 	spin_unlock_irqrestore(&svm_vm_data_hash_lock, flags);
 
-	if (!vcpu)
-		return 0;
-
 	/* Note:
 	 * At this point, the IOMMU should have already set the pending
 	 * bit in the vAPIC backing page. So, we just need to schedule
 	 * in the vcpu.
 	 */
-	if (vcpu->mode == OUTSIDE_GUEST_MODE)
+	if (vcpu)
 		kvm_vcpu_wake_up(vcpu);
 
 	return 0;
-- 
1.8.3.1

Re: [PATCH] KVM: SVM: unconditionally wake up VCPU on IOMMU interrupt

[Radim Krčmář]

2017-10-10 12:57+0200, Paolo Bonzini:
> Checking the mode is unnecessary, and is done without a memory barrier
> separating the LAPIC write from the vcpu->mode read; in addition,
> kvm_vcpu_wake_up is already doing a check for waiters on the wait queue
> that has the same effect.
> 
> In practice it's safe because spin_lock has full-barrier semantics on x86,
> but don't be too clever.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>