KVM: x86: update pvclock area conditionally, on cpu migration

As requested by Glauber, do not update kvmclock area on vcpu->pcpu
migration, in case the host has stable TSC.

This is to reduce cacheline bouncing.

Acked-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 422ef5ed2194..fd766214d9da 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2627,7 +2627,12 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
                         kvm_x86_ops->write_tsc_offset(vcpu, offset);
                         vcpu->arch.tsc_catchup = 1;
                 }
-                kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
+                /*
+                 * 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)
+                        kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
                 if (vcpu->cpu != cpu)
                         kvm_migrate_timers(vcpu);
                 vcpu->cpu = cpu;