Re: [PATCH] sched/cputime: add steal clock warps handling during cpu hotplug

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/06/2016 15:59, Rik van Riel wrote:
> If a guest is saved to disk and later restored (eg. after
> a host reboot), or live migrated to another host, I would
> expect to get totally disjoint steal time statistics from
> the "new run" of the guest (which is the same run of the
> guest OS).

Why?  The preexisting guest steal time is always added to by
KVM, so the time won't restart from zero.

Continuing the previous count on CPU hot-unplug followed by hot-plug
is less obvious, but I think it's overall the right thing to do.

In fact, I was going to test a patch this week as simple as this:

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index eea2a6f72b31..1ef5e48b3a36 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -301,8 +301,6 @@ static void kvm_register_steal_time(void)
 	if (!has_steal_clock)
 		return;
 
-	memset(st, 0, sizeof(*st));
-
 	wrmsrl(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED));
 	pr_info("kvm-stealtime: cpu %d, msr %llx\n",
 		cpu, (unsigned long long) slow_virt_to_phys(st));


Thanks,

Paolo

> In fact, this code may also need to deal with the case
> where steal time suddenly increases by a ludicrous amount,
> and ignore those events, too.
> 
> A safe threshold might be to only apply steal times that
> are positive and smaller than one second (as long as nohz_full
> has the one second timer tick left), ignoring intervals that
> are negative or longer than a second, and using those to sync
> up the guest with the host.