[PATCH v4 24/30] KVM: x86: Avoid gratuitous global clock updates

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

Eliminate three sources of unnecessary KVM_REQ_GLOBAL_CLOCK_UPDATE:

1. kvm_write_system_time(): The global clock update was a workaround for
   ever-drifting clocks based on the host's CLOCK_MONOTONIC subject to
   NTP skew. On booting or resuming a guest, it just leads to running
   kvm_guest_time_update() twice for each vCPU for no good reason. Use
   KVM_REQ_CLOCK_UPDATE on the vCPU itself, and only when the clock is
   being enabled, not disabled.

2. kvm_arch_vcpu_load(): Use KVM_REQ_CLOCK_UPDATE instead of
   KVM_REQ_GLOBAL_CLOCK_UPDATE. There is no need to update all vCPUs'
   clocks when one vCPU is loaded.

3. kvm_gen_kvmclock_update(): Skip the periodic global update entirely
   when in master clock mode, since the clock is defined precisely by
   the guest TSC and doesn't drift.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
---
 arch/x86/kvm/x86.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0656d901fe79..7d9ec0638d28 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2457,13 +2457,13 @@ static void kvm_write_system_time(struct kvm_vcpu *vcpu, gpa_t system_time,
 	}
 
 	vcpu->arch.time = system_time;
-	kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
 
 	/* we verify if the enable bit is set... */
-	if (system_time & 1)
+	if (system_time & 1) {
 		kvm_gpc_activate(&vcpu->arch.pv_time, system_time & ~1ULL,
 				 sizeof(struct pvclock_vcpu_time_info));
-	else
+		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
+	} else
 		kvm_gpc_deactivate(&vcpu->arch.pv_time);
 
 	return;
@@ -3638,6 +3638,10 @@ static void kvm_gen_kvmclock_update(struct kvm_vcpu *v)
 	struct kvm_vcpu *vcpu;
 	struct kvm *kvm = v->kvm;
 
+	/* In master clock mode, the clock doesn't need periodic updates. */
+	if (kvm->arch.use_master_clock)
+		return;
+
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
 		kvm_vcpu_kick(vcpu);
@@ -5339,7 +5343,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		 * kvmclock on vcpu->cpu migration
 		 */
 		if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1)
-			kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
+			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
 		if (vcpu->cpu != cpu)
 			kvm_make_request(KVM_REQ_MIGRATE_TIMER, vcpu);
 		vcpu->cpu = cpu;
-- 
2.51.0