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authorAndrew Jones <drjones@redhat.com>2014-02-28 12:52:54 +0100
committerPaolo Bonzini <pbonzini@redhat.com>2014-03-04 11:50:47 +0100
commit7e44e4495a398eb553ce561f29f9148f40a3448f (patch)
tree874a17c193b6be9621fbc32ba7c7ce633e316658 /arch/x86/kvm
parentccf9844e5d99c1ee9a5b8c4f1332ac5211cbce03 (diff)
downloadop-kernel-dev-7e44e4495a398eb553ce561f29f9148f40a3448f.zip
op-kernel-dev-7e44e4495a398eb553ce561f29f9148f40a3448f.tar.gz
x86: kvm: rate-limit global clock updates
When we update a vcpu's local clock it may pick up an NTP correction. We can't wait an indeterminate amount of time for other vcpus to pick up that correction, so commit 0061d53daf26f introduced a global clock update. However, we can't request a global clock update on every vcpu load either (which is what happens if the tsc is marked as unstable). The solution is to rate-limit the global clock updates. Marcelo calculated that we should delay the global clock updates no more than 0.1s as follows: Assume an NTP correction c is applied to one vcpu, but not the other, then in n seconds the delta of the vcpu system_timestamps will be c * n. If we assume a correction of 500ppm (worst-case), then the two vcpus will diverge 50us in 0.1s, which is a considerable amount. Signed-off-by: Andrew Jones <drjones@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'arch/x86/kvm')
-rw-r--r--arch/x86/kvm/x86.c27
1 files changed, 23 insertions, 4 deletions
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 773eba7..5ed9293 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1628,14 +1628,21 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
* 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.
+ * We need to rate-limit these requests though, as they can
+ * considerably slow guests that have a large number of vcpus.
+ * The time for a remote vcpu to update its kvmclock is bound
+ * by the delay we use to rate-limit the updates.
*/
-static void kvm_gen_kvmclock_update(struct kvm_vcpu *v)
+#define KVMCLOCK_UPDATE_DELAY msecs_to_jiffies(100)
+
+static void kvmclock_update_fn(struct work_struct *work)
{
int i;
- struct kvm *kvm = v->kvm;
+ struct delayed_work *dwork = to_delayed_work(work);
+ struct kvm_arch *ka = container_of(dwork, struct kvm_arch,
+ kvmclock_update_work);
+ struct kvm *kvm = container_of(ka, struct kvm, arch);
struct kvm_vcpu *vcpu;
kvm_for_each_vcpu(i, vcpu, kvm) {
@@ -1644,6 +1651,15 @@ static void kvm_gen_kvmclock_update(struct kvm_vcpu *v)
}
}
+static void kvm_gen_kvmclock_update(struct kvm_vcpu *v)
+{
+ struct kvm *kvm = v->kvm;
+
+ set_bit(KVM_REQ_CLOCK_UPDATE, &v->requests);
+ schedule_delayed_work(&kvm->arch.kvmclock_update_work,
+ KVMCLOCK_UPDATE_DELAY);
+}
+
static bool msr_mtrr_valid(unsigned msr)
{
switch (msr) {
@@ -7022,6 +7038,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
pvclock_update_vm_gtod_copy(kvm);
+ INIT_DELAYED_WORK(&kvm->arch.kvmclock_update_work, kvmclock_update_fn);
+
return 0;
}
@@ -7059,6 +7077,7 @@ static void kvm_free_vcpus(struct kvm *kvm)
void kvm_arch_sync_events(struct kvm *kvm)
{
+ cancel_delayed_work_sync(&kvm->arch.kvmclock_update_work);
kvm_free_all_assigned_devices(kvm);
kvm_free_pit(kvm);
}
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