KVM: x86/xen: Prevent runstate times from becoming negative

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
runstate times until the KVM clock catches up with runstate_entry_time
again.

Also clamp steal_ns to delta_ns to prevent steal time from exceeding
the total elapsed time, and handle negative steal_ns (which can happen
if run_delay goes backwards across a scheduler update).

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>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://patch.msgid.link/20260728144954.355376-21-dwmw2@infradead.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

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