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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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