[PATCH] KVM: Improve interrupt response

The current interrupt injection mechanism might delay an interrupt under
the following circumstances:

 - if injection fails because the guest is not interruptible (rflags.IF clear,
   or after a 'mov ss' or 'sti' instruction).  Userspace can check rflags,
   but the other cases or not testable under the current API.
 - if injection fails because of a fault during delivery.  This probably
   never happens under normal guests.
 - if injection fails due to a physical interrupt causing a vmexit so that
   it can be handled by the host.

In all cases the guest proceeds without processing the interrupt, reducing
the interactive feel and interrupt throughput of the guest.

This patch fixes the situation by allowing userspace to request an exit
when the 'interrupt window' opens, so that it can re-inject the interrupt
at the right time.  Guest interactivity is very visibly improved.

Signed-off-by: Dor Laor <dor.laor@qumranet.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 87 insertions, 7 deletions

diff --git a/drivers/kvm/svm.c b/drivers/kvm/svm.c
index fa0428735717..855207a9b396 100644
--- a/drivers/kvm/svm.c
+++ b/drivers/kvm/svm.c
@@ -235,6 +235,8 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
 
 	vcpu->rip = vcpu->svm->vmcb->save.rip = vcpu->svm->next_rip;
 	vcpu->svm->vmcb->control.int_state &= ~SVM_INTERRUPT_SHADOW_MASK;
+
+	vcpu->interrupt_window_open = 1;
 }
 
 static int has_svm(void)
@@ -1031,10 +1033,11 @@ static int halt_interception(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 {
 	vcpu->svm->next_rip = vcpu->svm->vmcb->save.rip + 1;
 	skip_emulated_instruction(vcpu);
-	if (vcpu->irq_summary && (vcpu->svm->vmcb->save.rflags & X86_EFLAGS_IF))
+	if (vcpu->irq_summary)
 		return 1;
 
 	kvm_run->exit_reason = KVM_EXIT_HLT;
+	++kvm_stat.halt_exits;
 	return 0;
 }
 
@@ -1186,6 +1189,24 @@ static int msr_interception(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 		return rdmsr_interception(vcpu, kvm_run);
 }
 
+static int interrupt_window_interception(struct kvm_vcpu *vcpu,
+				   struct kvm_run *kvm_run)
+{
+	/*
+	 * If the user space waits to inject interrupts, exit as soon as
+	 * possible
+	 */
+	if (kvm_run->request_interrupt_window &&
+	    !vcpu->irq_summary &&
+	    (vcpu->svm->vmcb->save.rflags & X86_EFLAGS_IF)) {
+		++kvm_stat.irq_window_exits;
+		kvm_run->exit_reason = KVM_EXIT_IRQ_WINDOW_OPEN;
+		return 0;
+	}
+
+	return 1;
+}
+
 static int (*svm_exit_handlers[])(struct kvm_vcpu *vcpu,
 				      struct kvm_run *kvm_run) = {
 	[SVM_EXIT_READ_CR0]           		= emulate_on_interception,
@@ -1210,6 +1231,7 @@ static int (*svm_exit_handlers[])(struct kvm_vcpu *vcpu,
 	[SVM_EXIT_NMI]				= nop_on_interception,
 	[SVM_EXIT_SMI]				= nop_on_interception,
 	[SVM_EXIT_INIT]				= nop_on_interception,
+	[SVM_EXIT_VINTR]			= interrupt_window_interception,
 	/* [SVM_EXIT_CR0_SEL_WRITE]		= emulate_on_interception, */
 	[SVM_EXIT_CPUID]			= cpuid_interception,
 	[SVM_EXIT_HLT]				= halt_interception,
@@ -1278,15 +1300,11 @@ static void pre_svm_run(struct kvm_vcpu *vcpu)
 }
 
 
-static inline void kvm_try_inject_irq(struct kvm_vcpu *vcpu)
+static inline void kvm_do_inject_irq(struct kvm_vcpu *vcpu)
 {
 	struct vmcb_control_area *control;
 
-	if (!vcpu->irq_summary)
-		return;
-
 	control = &vcpu->svm->vmcb->control;
-
 	control->int_vector = pop_irq(vcpu);
 	control->int_ctl &= ~V_INTR_PRIO_MASK;
 	control->int_ctl |= V_IRQ_MASK |
@@ -1301,6 +1319,59 @@ static void kvm_reput_irq(struct kvm_vcpu *vcpu)
 		control->int_ctl &= ~V_IRQ_MASK;
 		push_irq(vcpu, control->int_vector);
 	}
+
+	vcpu->interrupt_window_open =
+		!(control->int_state & SVM_INTERRUPT_SHADOW_MASK);
+}
+
+static void do_interrupt_requests(struct kvm_vcpu *vcpu,
+				       struct kvm_run *kvm_run)
+{
+	struct vmcb_control_area *control = &vcpu->svm->vmcb->control;
+
+	vcpu->interrupt_window_open =
+		(!(control->int_state & SVM_INTERRUPT_SHADOW_MASK) &&
+		 (vcpu->svm->vmcb->save.rflags & X86_EFLAGS_IF));
+
+	if (vcpu->interrupt_window_open && vcpu->irq_summary)
+		/*
+		 * If interrupts enabled, and not blocked by sti or mov ss. Good.
+		 */
+		kvm_do_inject_irq(vcpu);
+
+	/*
+	 * Interrupts blocked.  Wait for unblock.
+	 */
+	if (!vcpu->interrupt_window_open &&
+	    (vcpu->irq_summary || kvm_run->request_interrupt_window)) {
+		control->intercept |= 1ULL << INTERCEPT_VINTR;
+	} else
+		control->intercept &= ~(1ULL << INTERCEPT_VINTR);
+}
+
+static void post_kvm_run_save(struct kvm_vcpu *vcpu,
+			      struct kvm_run *kvm_run)
+{
+	kvm_run->ready_for_interrupt_injection = (vcpu->interrupt_window_open &&
+						  vcpu->irq_summary == 0);
+	kvm_run->if_flag = (vcpu->svm->vmcb->save.rflags & X86_EFLAGS_IF) != 0;
+	kvm_run->cr8 = vcpu->cr8;
+	kvm_run->apic_base = vcpu->apic_base;
+}
+
+/*
+ * Check if userspace requested an interrupt window, and that the
+ * interrupt window is open.
+ *
+ * No need to exit to userspace if we already have an interrupt queued.
+ */
+static int dm_request_for_irq_injection(struct kvm_vcpu *vcpu,
+					  struct kvm_run *kvm_run)
+{
+	return (!vcpu->irq_summary &&
+		kvm_run->request_interrupt_window &&
+		vcpu->interrupt_window_open &&
+		(vcpu->svm->vmcb->save.rflags & X86_EFLAGS_IF));
 }
 
 static void save_db_regs(unsigned long *db_regs)
@@ -1326,7 +1397,7 @@ static int svm_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	u16 ldt_selector;
 
 again:
-	kvm_try_inject_irq(vcpu);
+	do_interrupt_requests(vcpu, kvm_run);
 
 	clgi();
 
@@ -1487,17 +1558,26 @@ again:
 	if (vcpu->svm->vmcb->control.exit_code == SVM_EXIT_ERR) {
 		kvm_run->exit_type = KVM_EXIT_TYPE_FAIL_ENTRY;
 		kvm_run->exit_reason = vcpu->svm->vmcb->control.exit_code;
+		post_kvm_run_save(vcpu, kvm_run);
 		return 0;
 	}
 
 	if (handle_exit(vcpu, kvm_run)) {
 		if (signal_pending(current)) {
 			++kvm_stat.signal_exits;
+			post_kvm_run_save(vcpu, kvm_run);
+			return -EINTR;
+		}
+
+		if (dm_request_for_irq_injection(vcpu, kvm_run)) {
+			++kvm_stat.request_irq_exits;
+			post_kvm_run_save(vcpu, kvm_run);
 			return -EINTR;
 		}
 		kvm_resched(vcpu);
 		goto again;
 	}
+	post_kvm_run_save(vcpu, kvm_run);
 	return 0;
 }