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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.07.16 at 17:39, <andrew.cooper3@xxxxxxxxxx> wrote:
> 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?

verify_tsc_reliability() may result in X86_FEATURE_TSC_RELIABLE
getting cleared. That's an initcall, which means it runs after
init_xen_time(), and hence after the rendezvous function got
established initially.

Jan


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