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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.01.2020 13:28, Igor Druzhinin wrote:
> On 15/01/2020 11:32, Jan Beulich wrote:
>> On 14.01.2020 20:36, Igor Druzhinin wrote:
>>> If ITSC is not available on CPU (e.g if running nested as PV shim)
>>> then X86_FEATURE_NONSTOP_TSC is not advertised in certain cases, i.e.
>>> all AMD and some old Intel processors. In which case TSC would need to
>>> be restored on CPU from platform time by Xen upon exiting deep C-states.
>>
>> How does waking from deep C states correspond to the PV shim? I notice
>> that cstate_restore_tsc() gets called irrespective of the C state being
>> exited, so I wonder whether there's room for improvement there
>> independent of the issue at hand. As far as this change is concerned,
>> I think you want to drop the notion of "deep" from the description.
> 
> I'm not familiar with what to call "deep C-state" so for me it was anything
> higher than C1. If you prefer "deep" to be dropped - so be it.

"Higher than C1" may be fine (albeit I vaguely recall TSC issues
starting with C3 only), but at least mwait_idle() calls the
function even for C1. As to the PV shim - does it know about any
C-states at all (beyond HLT-invoked C1)?

Jan

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