[RFC PATCH v3 20/21] KVM: x86/xen: Prevent runstate times from becoming negative

[David Woodhouse]

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