Re: [PATCH v14 20/25] x86/tsc: calibrate tsc only once

Pavel Tatashin
2018-07-19T20:46:43Z

On 07/19/2018 04:44 PM, Thomas Gleixner wrote:
> On Thu, 19 Jul 2018, Pavel Tatashin wrote:
>
>> On Thu, Jul 19, 2018 at 12:49 PM Pavel Tatashin
>> <pasha.tatashin@oracle.com> wrote:
>>>
>>>> So create two functions. native_...early..() and native....(). The early
>>>> one does not contain the hpet/pmtimer stuff and it replaces the ops.pointer
>>>> with the late one which contains all of it.
>>>
>>> Good idea. Actually, the late one will contain only hpet/pmtimer and I
>>> will set it only if tsc frequency was not determined only.
>>
>> If we determined tsc early in boot using one of the quick methods:
>> from cpuid/msr/quick_pit, can we assume that frequencies of all other
>> CPUs will be determined the same way? Or do we still have to fallback
>> to PIT/HPET/PMTIMER? I wondering if we support heterogeneous
>> multi-socket platforms with different CPUs, because that the only
>> platforms where I see such scenario is possible.
>
> The frequency for secondary CPUs is usually taken from the boot CPU and the
> only reason why recalibration can happen is when the CPU does not have a
> constant frequency TSC.
>
> For that case the quick PIT + hpet/pmtimer calibration bundle is
> required. So yes, the early calibration might work with quick PIT (those
> CPUs definitely do not have MSR/CPUID based calibration), but the
> recalibration might fail the quick PIT calibration for various reasons.

OK, good, I implemented with this in-mind. I will send out a new series shortly.

Thank you,
Pavel

>
> Thanks,
>
> tglx
>