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Commit 7e44e449 authored by Andrew Jones's avatar Andrew Jones Committed by Paolo Bonzini
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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 0061d53d 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: default avatarAndrew Jones <drjones@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent ccf9844e

arch/x86/include/asm/kvm_host.h

+1 −0
Original line number Diff line number Diff line
@@ -598,6 +598,7 @@ struct kvm_arch { 
	bool use_master_clock;

	u64 master_kernel_ns;

	cycle_t master_cycle_now;

	struct delayed_work kvmclock_update_work;



	struct kvm_xen_hvm_config xen_hvm_config;

arch/x86/kvm/x86.c

+23 −4
Original line number Diff line number Diff line
@@ -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);

}