KVM: x86: limit difference between kvmclock updates

kvmclock updates which are isolated to a given vcpu, such as vcpu->cpu
migration, should not allow system_timestamp from the rest of the vcpus
to remain static. Otherwise ntp frequency correction applies to one
vcpu's system_timestamp but not the others.

So in those cases, request a kvmclock update for all vcpus. The worst
case for a remote vcpu to update its kvmclock is then bounded by maximum
nohz sleep latency.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

1 files changed, 28 insertions, 2 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 094b5d96ab14..8d28810a5f88 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1588,6 +1588,30 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
         return 0;
 }
 
+/*
+ * kvmclock updates which are isolated to a given vcpu, such as
+ * vcpu->cpu migration, should not allow system_timestamp from
+ * the rest of the vcpus to remain static. Otherwise ntp frequency
+ * correction applies to one vcpu's system_timestamp but not
+ * the others.
+ *
+ * So in those cases, request a kvmclock update for all vcpus.
+ * The worst case for a remote vcpu to update its kvmclock
+ * is then bounded by maximum nohz sleep latency.
+ */
+
+static void kvm_gen_kvmclock_update(struct kvm_vcpu *v)
+{
+        int i;
+        struct kvm *kvm = v->kvm;
+        struct kvm_vcpu *vcpu;
+
+        kvm_for_each_vcpu(i, vcpu, kvm) {
+                set_bit(KVM_REQ_CLOCK_UPDATE, &vcpu->requests);
+                kvm_vcpu_kick(vcpu);
+        }
+}
+
 static bool msr_mtrr_valid(unsigned msr)
 {
         switch (msr) {
@@ -1985,7 +2009,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
                 kvmclock_reset(vcpu);
 
                 vcpu->arch.time = data;
-                kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
+                kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
 
                 /* we verify if the enable bit is set... */
                 if (!(data & 1))
@@ -2702,7 +2726,7 @@ 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_CLOCK_UPDATE, vcpu);
+                        kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
                 if (vcpu->cpu != cpu)
                         kvm_migrate_timers(vcpu);
                 vcpu->cpu = cpu;
@@ -5703,6 +5727,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
                         __kvm_migrate_timers(vcpu);
                 if (kvm_check_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu))
                         kvm_gen_update_masterclock(vcpu->kvm);
+                if (kvm_check_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu))
+                        kvm_gen_kvmclock_update(vcpu);
                 if (kvm_check_request(KVM_REQ_CLOCK_UPDATE, vcpu)) {
                         r = kvm_guest_time_update(vcpu);
                         if (unlikely(r))