On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
>
> 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@amazon.co.uk>
> ---
> 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).
> + */
I take it this is a legitimate scenario where userpace sets KVM clock and then
the runstates, and KVM needs to lend a hand because userspace can't do those two
things atomically?
> + 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
>