[PATCH v3] KVM: LAPIC: Fix lapic timer injection delay

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

If the TSC deadline timer is programmed really close to the deadline or 
even in the past, the computation in vmx_set_hv_timer will program the 
absolute target tsc value to vmcs preemption timer field w/ delta == 0, 
then plays a vmentry and an upcoming vmx preemption timer fire vmexit 
dance, the lapic timer injection is delayed due to this duration. Actually 
the lapic timer which is emulated by hrtimer can handle this correctly.

This patch fixes it by firing the lapic timer and injecting a timer interrupt 
immediately during the next vmentry if the TSC deadline timer is programmed
really close to the deadline or even in the past. This saves ~300 cycles on 
the tsc_deadline_timer test of apic.flat.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v2 -> v3:
 * update the patch description

 arch/x86/kvm/lapic.c | 7 ++++++-
 arch/x86/kvm/vmx.c   | 3 +++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index d24c874..08cf73d 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1504,12 +1504,17 @@ static void cancel_hv_timer(struct kvm_lapic *apic)
 static bool start_hv_timer(struct kvm_lapic *apic)
 {
 	u64 tscdeadline = apic->lapic_timer.tscdeadline;
+	int ret = 0;
 
 	if ((atomic_read(&apic->lapic_timer.pending) &&
 		!apic_lvtt_period(apic)) ||
-		kvm_x86_ops->set_hv_timer(apic->vcpu, tscdeadline)) {
+		(ret = kvm_x86_ops->set_hv_timer(apic->vcpu, tscdeadline))) {
 		if (apic->lapic_timer.hv_timer_in_use)
 			cancel_hv_timer(apic);
+		if (ret == 1) {
+			apic_timer_expired(apic);
+			return true;
+		}
 	} else {
 		apic->lapic_timer.hv_timer_in_use = true;
 		hrtimer_cancel(&apic->lapic_timer.timer);
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 2e906cf..2008e9b 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -11149,6 +11149,9 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
 	u64 guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
 	u64 delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
 
+	if (delta_tsc == 0)
+		return 1;
+
 	/* Convert to host delta tsc if tsc scaling is enabled */
 	if (vcpu->arch.tsc_scaling_ratio != kvm_default_tsc_scaling_ratio &&
 			u64_shl_div_u64(delta_tsc,
-- 
2.7.4