[PATCH 2/2] KVM: SVM: check for progress after IRET interception

[Avi Kivity <avi@redhat.com>]

When we enable an NMI window, we ask for an IRET intercept, since
the IRET re-enables NMIs.  However, the IRET intercept happens before
the instruction executes, while the NMI window architecturally opens
afterwards.

To compensate for this mismatch, we only open the NMI window in the
following exit, assuming that the IRET has by then executed; however,
this assumption is not always correct; we may exit due to a host interrupt
or page fault, without having executed the instruction.

Fix by checking for forward progress by recording and comparing the IRET's
rip.  This is somewhat of a hack, since an unchaging rip does not mean that
no forward progress has been made, but is the simplest fix for now.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 arch/x86/kvm/svm.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 73a8f1d..53c5d8a 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -135,6 +135,8 @@ struct vcpu_svm {
 
 	u32 *msrpm;
 
+	ulong nmi_iret_rip;
+
 	struct nested_state nested;
 
 	bool nmi_singlestep;
@@ -2653,6 +2655,7 @@ static int iret_interception(struct vcpu_svm *svm)
 	++svm->vcpu.stat.nmi_window_exits;
 	clr_intercept(svm, INTERCEPT_IRET);
 	svm->vcpu.arch.hflags |= HF_IRET_MASK;
+	svm->nmi_iret_rip = kvm_rip_read(&svm->vcpu);
 	return 1;
 }
 
@@ -3472,7 +3475,12 @@ static void svm_complete_interrupts(struct vcpu_svm *svm)
 
 	svm->int3_injected = 0;
 
-	if (svm->vcpu.arch.hflags & HF_IRET_MASK) {
+	/*
+	 * If we've made progress since setting HF_IRET_MASK, we've
+	 * executed an IRET and can allow NMI injection.
+	 */
+	if ((svm->vcpu.arch.hflags & HF_IRET_MASK)
+	    && kvm_rip_read(&svm->vcpu) != svm->nmi_iret_rip) {
 		svm->vcpu.arch.hflags &= ~(HF_NMI_MASK | HF_IRET_MASK);
 		kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);
 	}
-- 
1.7.1