[PATCH v4 7/7] KVM: VMX: Handle preemption timer fastpath

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

This patch implements handle preemption timer fastpath, after timer fire
due to VMX-preemption timer counts down to zero, handle it as soon as
possible and vmentry immediately without checking various kvm stuff when
possible.

Testing on SKX Server.

cyclictest in guest(w/o mwait exposed, adaptive advance lapic timer is default -1):

5540.5ns -> 4602ns       17%

kvm-unit-test/vmexit.flat:

w/o avanced timer:
tscdeadline_immed: 3028.5  -> 2494.75  17.6%
tscdeadline:       5765.7  -> 5285      8.3%

w/ adaptive advance timer default -1:
tscdeadline_immed: 3123.75 -> 2583     17.3%
tscdeadline:       4663.75 -> 4537      2.7%

Tested-by: Haiwei Li <lihaiwei@tencent.com>
Cc: Haiwei Li <lihaiwei@tencent.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/vmx/vmx.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index bb5c4f1..7dcc99f 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5556,17 +5556,34 @@ static int handle_pml_full(struct kvm_vcpu *vcpu)
 	return 1;
 }
 
-static int handle_preemption_timer(struct kvm_vcpu *vcpu)
+static int __handle_preemption_timer(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
 	if (!vmx->req_immediate_exit &&
-	    !unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled))
+	    !unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled)) {
 		kvm_lapic_expired_hv_timer(vcpu);
+		return 1;
+	}
 
+	return 0;
+}
+
+static int handle_preemption_timer(struct kvm_vcpu *vcpu)
+{
+	__handle_preemption_timer(vcpu);
 	return 1;
 }
 
+static fastpath_t handle_fastpath_preemption_timer(struct kvm_vcpu *vcpu)
+{
+	if (__handle_preemption_timer(vcpu)) {
+		trace_kvm_exit(EXIT_REASON_PREEMPTION_TIMER, vcpu, KVM_ISA_VMX);
+		return EXIT_FASTPATH_REENTER_GUEST;
+	}
+	return EXIT_FASTPATH_NONE;
+}
+
 /*
  * When nested=0, all VMX instruction VM Exits filter here.  The handlers
  * are overwritten by nested_vmx_setup() when nested=1.
@@ -6590,6 +6607,8 @@ static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
 		switch (to_vmx(vcpu)->exit_reason) {
 		case EXIT_REASON_MSR_WRITE:
 			return handle_fastpath_set_msr_irqoff(vcpu);
+		case EXIT_REASON_PREEMPTION_TIMER:
+			return handle_fastpath_preemption_timer(vcpu);
 		default:
 			return EXIT_FASTPATH_NONE;
 		}
-- 
2.7.4