[PATCH 3/3] KVM: SVM: Short circuit STI; HLT while an interrupt is pending

[Avi Kivity <avi@redhat.com>]

Short-circuit an STI; HLT sequence while an interrupt is pending:
instead of halting, re-entering the guest, and exiting immediately
on an interrupt window exit, go directly to the last step.

Saves a vmexit on workloads where interrupts are received synchronously;
an example is a disk backed by the host page cache where there is no
latency (from the guest's point of view) between the request and fulfilment.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 arch/x86/kvm/svm.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 53c5d8a..b70af8b 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -184,6 +184,7 @@ module_param(nested, int, S_IRUGO);
 
 static void svm_flush_tlb(struct kvm_vcpu *vcpu);
 static void svm_complete_interrupts(struct vcpu_svm *svm);
+static int interrupt_window_interception(struct vcpu_svm *svm);
 
 static int nested_svm_exit_handled(struct vcpu_svm *svm);
 static int nested_svm_intercept(struct vcpu_svm *svm);
@@ -1745,6 +1746,14 @@ static int halt_interception(struct vcpu_svm *svm)
 {
 	svm->next_rip = kvm_rip_read(&svm->vcpu) + 1;
 	skip_emulated_instruction(&svm->vcpu);
+	/*
+	 * Short-circuit an STI; HLT sequence while an interrupt is pending:
+	 * instead of halting, re-entering the guest, and exiting immediately
+	 * on an interrupt window exit, go directly to the last step.
+	 */
+	if ((svm->vmcb->control.intercept & (1ULL << INTERCEPT_VINTR))
+	    && (kvm_get_rflags(&svm->vcpu) & X86_EFLAGS_IF))
+		return interrupt_window_interception(svm);
 	return kvm_emulate_halt(&svm->vcpu);
 }
 
-- 
1.7.1