[PATCH v5 24/34] KVM: x86: Avoid gratuitous global clock updates

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

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@xxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 72fb4620a5ba..4fc21d701588 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;
@@ -5377,8 +5377,10 @@ 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)
                        kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
+               else if (vcpu->cpu == -1)
+                       kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
                if (vcpu->cpu != cpu)
                        kvm_make_request(KVM_REQ_MIGRATE_TIMER, vcpu);
                vcpu->cpu = cpu;
-- 
2.54.0