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

[Paul Durrant]

On 22/05/2024 01:17, David Woodhouse wrote:
> 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(-)

Reviewed-by: Paul Durrant <paul@xxxxxxx>