From: David Woodhouse <dwmw@xxxxxxxxxxxx> When kvm_xen_update_runstate() is invoked to set a vCPU's runstate, the time spent in the previous runstate is accounted. This is based on the delta between the current KVM clock time, and the previous value stored in vcpu->arch.xen.runstate_entry_time. If the KVM clock goes backwards, that delta will be negative. Or, since it's an unsigned 64-bit integer, very *large*. Linux guests deal with that particularly badly, reporting 100% steal time for ever more (well, for *centuries* at least, until the delta has been consumed). So when a negative delta is detected, just refrain from updating the runstates until the KVM clock catches up with runstate_entry_time again. The userspace APIs for setting the runstate times do not allow them to be set past the current KVM clock, but userspace can still adjust the KVM clock *after* setting the runstate times, which would cause this situation to occur. Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx> --- arch/x86/kvm/xen.c | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-) diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c index 014048c22652..3d4111de4472 100644 --- a/arch/x86/kvm/xen.c +++ b/arch/x86/kvm/xen.c @@ -538,24 +538,34 @@ void kvm_xen_update_runstate(struct kvm_vcpu *v, int state) { struct kvm_vcpu_xen *vx = &v->arch.xen; u64 now = get_kvmclock_ns(v->kvm); - u64 delta_ns = now - vx->runstate_entry_time; u64 run_delay = current->sched_info.run_delay; + s64 delta_ns = now - vx->runstate_entry_time; + s64 steal_ns = run_delay - vx->last_steal; if (unlikely(!vx->runstate_entry_time)) vx->current_runstate = RUNSTATE_offline; + vx->last_steal = run_delay; + + /* + * If KVM clock time went backwards, stop updating until it + * catches up (or the runstates are reset by userspace). + */ + if (delta_ns < 0) + return; + /* * Time waiting for the scheduler isn't "stolen" if the * vCPU wasn't running anyway. */ - if (vx->current_runstate == RUNSTATE_running) { - u64 steal_ns = run_delay - vx->last_steal; + if (vx->current_runstate == RUNSTATE_running && steal_ns > 0) { + if (steal_ns > delta_ns) + steal_ns = delta_ns; delta_ns -= steal_ns; vx->runstate_times[RUNSTATE_runnable] += steal_ns; } - vx->last_steal = run_delay; vx->runstate_times[vx->current_runstate] += delta_ns; vx->current_runstate = state; -- 2.44.0