Re: [Xen-devel] [PATCH 5/8] x86/time: correctly honor late clearing of TSC related feature flags

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/06/16 16:20, Jan Beulich wrote:
>>>> On 20.06.16 at 16:32, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 15/06/16 11:28, Jan Beulich wrote:
>>> --- a/xen/arch/x86/time.c
>>> +++ b/xen/arch/x86/time.c
>>> @@ -1358,6 +1358,24 @@ static void time_calibration(void *unuse
>>>                       &r, 1);
>>>  }
>>>  
>>> +void __init clear_tsc_cap(unsigned int feature)
>>> +{
>>> +    void (*rendezvous_fn)(void *) = time_calibration_std_rendezvous;
>> This should read time_calibration_rendezvous_fn rather than assuming
>> time_calibration_std_rendezvous.
>>
>> Otherwise, there is a risk that it could be reset back to
>> time_calibration_std_rendezvous.
> But that's the purpose: We may need to switch back.

Under what circumstances could we ever move from re-syncing back to not
re-syncing?

Even in a scenario with pcpu hotplug, where we lose the final pcpu which
was causing re-syncing, we don't know that one of the intermediate pcpus
hasn't gone and come back, with a different TSC base.

~Andrew

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