Re: [Xen-devel] [PATCH v3] x86/hvm/hpet: avoid 'small' time diff test on resume

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.05.19 at 16:07, <paul.durrant@xxxxxxxxxx> wrote:
> It appears that even 64-bit versions of Windows 10, when not using syth-
> etic timers, will use 32-bit HPET non-periodic timers. There is a test
> in hpet_set_timer(), specific to 32-bit timers, that tries to disambiguate
> between a comparator value that is in the past and one that is sufficiently
> far in the future that it wraps. This is done by assuming that the delta
> between the main counter and comparator will be 'small' [1], if the
> comparator value is in the past. Unfortunately, more often than not, this
> is not the case if the timer is being re-started after a migrate and so
> the timer is set to fire far in the future (in excess of a minute in
> several observed cases) rather then set to fire immediately. This has a
> rather odd symptom where the guest console is alive enough to be able to
> deal with mouse pointer re-rendering, but any keyboard activity or mouse
> clicks yield no response.
> 
> This patch simply adds an extra check of 'creation_finished' into
> hpet_set_timer() so that the 'small' time test is omitted when the function
> is called to restart timers after migration, and thus any negative delta
> causes a timer to fire immediately.
> 
> [1] The number of ticks that equate to 0.9765625 milliseconds
> 
> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> I notice that we seemingly don't handle main counter wrap in the HPET code.
> The spec. says that timers should fire at the point the counter wraps at the
> timer's width. I think the need for the 'small' time test would go away if
> this was implemented, but that's for another day.

For posterity I would have hoped you would update this remark as
well.

Jan



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