Re: [Xen-devel] [PATCH v2] x86/time: Don't use virtual TSC if host and guest frequencies are equal

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/17/2017 10:24 AM, Jan Beulich wrote:
>>>> On 17.03.17 at 14:36, <boris.ostrovsky@xxxxxxxxxx> wrote:
>> On 03/17/2017 03:48 AM, Jan Beulich wrote:
>>>>>> On 16.03.17 at 20:35, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>> --- a/xen/arch/x86/time.c
>>>> +++ b/xen/arch/x86/time.c
>>>> @@ -2051,17 +2051,12 @@ void tsc_set_info(struct domain *d,
>>>>          d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
>>>>          d->arch.tsc_khz = gtsc_khz ?: cpu_khz;
>>>>          set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000);
>>>> -        /*
>>>> -         * In default mode use native TSC if the host has safe TSC and:
>>>> -         *  HVM/PVH: host and guest frequencies are the same (either
>>>> -         *           "naturally" or via TSC scaling)
>>>> -         *  PV: guest has not migrated yet (and thus arch.tsc_khz == 
>>>> cpu_khz)
>>>> -         */
>>>> +
>>>> +        ASSERT(incarnation || d->arch.tsc_khz == cpu_khz);
>>> Hmm, is this valid for other than TSC_MODE_DEFAULT?
>> It is valid for all modes but I thought that the ASSERT is really only
>> "interesting" for DEFAULT and ALWAYS_EMULATE since this is when we
>> decide whether or not to set vtsc.
>>
>> Since I need to rebase this anyway (due to PVH1 removal) I can move this
>> down right after the switch if you feel it would be useful.
> Actually I think the other way around: For ALWAYS_EMULATE as
> well as for PVRDTSCP I don't think the assertion is valid, the more
> that d->arch.tsc_khz gets set from input to the function. That last
> fact actually makes the ASSERT() dubious in all cases, I'm afraid.

It is valid (in the sense that it will evaluate to true) because we
always first call tsc_set_info with DEFAULT mode and with gtsc_khz=0
from arch_domain_create(). So d->arch.tsc_khz will be primed to cpu_khz.

-boris


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