[PATCH 2/3] KVM: VMX: 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/vmx.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index ee1cd1a..541da0e 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3437,6 +3437,15 @@ static int handle_interrupt_window(struct kvm_vcpu *vcpu)
 static int handle_halt(struct kvm_vcpu *vcpu)
 {
 	skip_emulated_instruction(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 ((to_vmx(vcpu)->cpu_based_vm_exec_control
+	     & CPU_BASED_VIRTUAL_INTR_PENDING)
+	    && (kvm_get_rflags(vcpu) & X86_EFLAGS_IF))
+		return handle_interrupt_window(vcpu);
 	return kvm_emulate_halt(vcpu);
 }
 
-- 
1.7.1