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x86: kvm: rate-limit global clock updates

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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 0061d53daf 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>
This commit is contained in:
Andrew Jones 2014-02-28 12:52:54 +01:00 committed by Paolo Bonzini
parent ccf9844e5d
commit 7e44e4495a
2 changed files with 24 additions and 4 deletions
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1
arch/x86/include/asm/kvm_host.h
View File

@ -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;

27
arch/x86/kvm/x86.c
View File

@ -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);
}