[PATCH 2/3] KVM: VMX: modify preemption timer bit only when arming timer

[Sean Christopherson <sean.j.christopherson@xxxxxxxxx>]

Provide a singular location where the VMX preemption timer bit is
set/cleared so that future usages of the preemption timer can ensure
the VMCS bit is up-to-date without having to modify unrelated code
paths.  For example, the preemption timer can be used to force an
immediate VMExit.  Cache the status of the timer to avoid redundant
VMREAD and VMWRITE, e.g. if the timer stays armed across multiple
VMEnters/VMExits.

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/kvm/vmx.c | 45 ++++++++++++++++++++++-----------------------
 1 file changed, 22 insertions(+), 23 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 04afaaeb27a7..5ae46af2077d 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1025,6 +1025,7 @@ struct vcpu_vmx {
 
 	/* apic deadline value in host tsc */
 	u64 hv_deadline_tsc;
+	bool hv_timer_armed;
 
 	u64 current_tsc_ratio;
 
@@ -10601,21 +10602,30 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
 static void vmx_arm_hv_timer(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
+	bool arm_timer;
 	u64 tscl;
 	u32 delta_tsc;
 
-	if (vmx->hv_deadline_tsc == -1)
-		return;
+	if (vmx->hv_deadline_tsc != -1) {
+		tscl = rdtsc();
+		if (vmx->hv_deadline_tsc > tscl)
+			/* set_hv_timer ensures the delta fits in 32-bits */
+			delta_tsc = (u32)((vmx->hv_deadline_tsc - tscl) >>
+				cpu_preemption_timer_multi);
+		else
+			delta_tsc = 0;
 
-	tscl = rdtsc();
-	if (vmx->hv_deadline_tsc > tscl)
-		/* sure to be 32 bit only because checked on set_hv_timer */
-		delta_tsc = (u32)((vmx->hv_deadline_tsc - tscl) >>
-			cpu_preemption_timer_multi);
-	else
-		delta_tsc = 0;
+		vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, delta_tsc);
+	}
 
-	vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, delta_tsc);
+	arm_timer = (vmx->hv_deadline_tsc != -1);
+	if (arm_timer && !vmx->hv_timer_armed)
+		vmcs_set_bits(PIN_BASED_VM_EXEC_CONTROL,
+			PIN_BASED_VMX_PREEMPTION_TIMER);
+	else if (!arm_timer && vmx->hv_timer_armed)
+		vmcs_clear_bits(PIN_BASED_VM_EXEC_CONTROL,
+			PIN_BASED_VMX_PREEMPTION_TIMER);
+	vmx->hv_timer_armed = arm_timer;
 }
 
 static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
@@ -13236,12 +13246,7 @@ static void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
 	vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, vmx->msr_autoload.host.nr);
 	vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_autoload.guest.nr);
 	vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
-	if (vmx->hv_deadline_tsc == -1)
-		vmcs_clear_bits(PIN_BASED_VM_EXEC_CONTROL,
-				PIN_BASED_VMX_PREEMPTION_TIMER);
-	else
-		vmcs_set_bits(PIN_BASED_VM_EXEC_CONTROL,
-			      PIN_BASED_VMX_PREEMPTION_TIMER);
+
 	if (kvm_has_tsc_control)
 		decache_tsc_multiplier(vmx);
 
@@ -13445,18 +13450,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
 		return -ERANGE;
 
 	vmx->hv_deadline_tsc = tscl + delta_tsc;
-	vmcs_set_bits(PIN_BASED_VM_EXEC_CONTROL,
-			PIN_BASED_VMX_PREEMPTION_TIMER);
-
 	return delta_tsc == 0;
 }
 
 static void vmx_cancel_hv_timer(struct kvm_vcpu *vcpu)
 {
-	struct vcpu_vmx *vmx = to_vmx(vcpu);
-	vmx->hv_deadline_tsc = -1;
-	vmcs_clear_bits(PIN_BASED_VM_EXEC_CONTROL,
-			PIN_BASED_VMX_PREEMPTION_TIMER);
+	to_vmx(vcpu)->hv_deadline_tsc = -1;
 }
 #endif
 
-- 
2.18.0