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

[Sean Christopherson <seanjc@google.com>]

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
>