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Re: [Xen-devel] Xen clocksource and PV shim



On 31.01.2020 04:02, Igor Druzhinin wrote:
> On 30/01/2020 23:14, Igor Druzhinin wrote:
>> I was debugging constant freezes of PV shim on AMD hardware
>> after going out of a long suspend.

What is "suspend" here? S3? If so, ...

>> As it turned out the root cause
>> of this is platform time jumping forward to the amount of time
>> spent in suspended state. On Intel this issue is papered over
>> by CONSTANT_TSC being set which avoids CPU time sync with
>> platform time.
>>
>> Upon further examination it appears that jumping is baked
>> into the implementation of L0 Xen and there is no seemingly
>> straight forward way to extract stable continuous rate out
>> of what we have.
>>
>> I expect this is a known issue with Xen PV clock as I found
>> this almost immediately: https://wiki.debian.org/Xen/Clocksource
>> Currently I don't understand how in that case Xen clock source
>> could be suitable as a platform timer for nested Xen.
>>
>> Is my understanding of the situation correct? Could it be
>> fixed in L0 Xen or it's already backed into the ABI? Should
>> we keep Xen platform timer in the source code then? Does using
>> alternative clock source for PV shim make sense?
> 
> ... Ok, I seem to get lost in the weeds of timekeeping code -
> platform timer infrastructure is already prepared for this sort
> of scenario while exiting S3. I just need to call resume_platform_timer()
> from hypervisor_resume() or something similar. Patch will follow.

... why would time_resume() not be called? Oh, pv_shim_shutdown()
uses PV mechanisms to do the suspend/resume. I wonder what else,
besides time_resume(), is missing there.

Jan

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