From 7e44e4495a398eb553ce561f29f9148f40a3448f Mon Sep 17 00:00:00 2001 From: Andrew Jones Date: Fri, 28 Feb 2014 12:52:54 +0100 Subject: [PATCH] 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 Signed-off-by: Paolo Bonzini --- arch/x86/include/asm/kvm_host.h | 1 + arch/x86/kvm/x86.c | 27 +++++++++++++++++++++++---- 2 files changed, 24 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index e714f8c08ccf..9aa09d330a4b 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -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; diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 773eba799c45..5ed9293a696d 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); } -- 2.30.2