Re: [Xen-devel] [PATCH] x86/time: update TSC stamp on restore from deep C-state

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.01.2020 10:33, Roger Pau Monné wrote:
> On Wed, Jan 15, 2020 at 05:21:16PM +0100, Jan Beulich wrote:
>> On 15.01.2020 14:44, Roger Pau Monné wrote:
>>> On Wed, Jan 15, 2020 at 01:49:22PM +0100, Jan Beulich wrote:
>>>> What I'm then worried about is too
>>>> little progress observable by guests. The PV time protocol
>>>> ought to be fine in this regard (and consumers of raw TSC values
>>>> are on their own anyway), but wouldn't you need to update TSC
>>>> offsets of HVM guests in order to compensate for the elapsed
>>>> time?
>>>
>>> That will be done when the HVM vCPU gets scheduled in as part of the
>>> update_vcpu_system_time call AFAICT. cstate_restore_tsc will always be
>>> called with the idle vCPU context, and hence there's always going to
>>> be a vCPU switch before scheduling anything else.
>>
>> Which step would this be? All I see is a call to hvm_scale_tsc().
>> In time.c only tsc_set_info() calls hvm_set_tsc_offset().
> 
> My bad, I've mistaken the scaling with the offset.
> 
> Accounting for the offset in update_vcpu_system_time seems quite
> more complicated that just updating the TSC here, so:
> 
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

And then (preferably with "deep" dropped from the description,
if you, Igor, agree, and which can be done while committing)
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan

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