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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/08/16 10:43, Jan Beulich wrote:
>>>> On 02.08.16 at 21:08, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 04/07/16 16:53, Jan Beulich wrote:
>>>>>> 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.
>> Right, but that isn't important.
>>
>> There will never be a case where, once TSC_RELIABLE is cleared, it is
>> safe to revert back to std_rendezvous, even if TSC_RELIABLE is
>> subsequently re-set.
> You've got this backwards: TSC_RELIABLE may get _cleared_ late.

Quite - I haven't got this backwards.

> Nothing can ever set it late, due to the use of setup_clear_cpu_cap().
> Reverting back to time_calibration_std_rendezvous() would only be
> possible if CONSTANT_TSC got cleared late, ...

time_calibration_rendezvous_fn defaults to
time_calibration_std_rendezvous(), i.e. defaults to the assumption that
the TSCs are invariant.

We then later call clear_caps(TSC_RELIABLE), and the default changes to
time_calibration_tsc_rendezvous().

We then later call clear_tsc_cap(CONSTANT_TSC), or indeed that
CONSTANT_TSC was never set in the first place, and the default switches
back to time_calibration_std_rendezvous() because of the aformentioned bug.

Once the switch to time_calibration_tsc_rendezvous() is made, it is
never safe to switch back.

Therefore, your function must read time_calibration_rendezvous_fn and
not assume time_calibration_std_rendezvous().

~Andrew

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