[PATCH v11 06/23] KVM: x86: Avoid gratuitous global clock updates

[David Woodhouse <dwmw2@infradead.org>]

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

Eliminate two 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. Now that the KVM clock uses CLOCK_MONOTONIC_RAW, the clock
   does not drift with NTP corrections and there is no need to
   synchronize all vCPUs on boot or resume. Use KVM_REQ_CLOCK_UPDATE on
   the vCPU itself, and only when the clock is being enabled, not
   disabled.

2. kvm_arch_vcpu_load(): In master clock mode, migration between pCPUs
   does not require any clock update since the master clock reference is
   shared. Only request a local KVM_REQ_CLOCK_UPDATE for the vCPU's
   first-ever load (vcpu->cpu == -1) to generate initial pvclock params.
   In non-master-clock mode, keep the global update to synchronize all
   vCPUs.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kvm/msrs.c | 7 ++++---
 arch/x86/kvm/x86.c  | 4 +++-
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/msrs.c b/arch/x86/kvm/msrs.c
index e91650af44c2..a2dffa1c706f 100644
--- a/arch/x86/kvm/msrs.c
+++ b/arch/x86/kvm/msrs.c
@@ -1285,14 +1285,15 @@ 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;
 }
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4c3fce29e2af..e3618d82c099 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2702,11 +2702,13 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		 * On a host with synchronized TSC, there is no need to update
 		 * kvmclock on vcpu->cpu migration
 		 */
-		if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1) {
+		if (!vcpu->kvm->arch.use_master_clock) {
 			if (__ratelimit(&vcpu->kvm->arch.kvmclock_update_rs))
 				kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
 			else
 				kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
+		} else if (vcpu->cpu == -1) {
+			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
 		}
 
 		if (vcpu->cpu != cpu)
-- 
2.43.0