linux-kernel - [PATCH v2] KVM: LAPIC: Fix lapic timer injection delay

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Date:   Fri, 23 Jun 2017 04:59:50 -0700
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: [PATCH v2] KVM: LAPIC: Fix lapic timer injection delay

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

Linux as a guest will prevent to program the next event to the clock event 
device when the event is in the past. However, it is not guaranteed by all 
the guests, the guest like kvm-unit-tests/apic.flat will write current tsc 
value visible in guest to MSR_IA32_TSCDEADLINE, lapic timer which is emulated 
by vmx preemption 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 
for 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 guest programs an expired timer to 
the emulated timer device. This saves ~300 cycles on the tsc_deadline_timer
test of apic.flat.

Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
---
 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