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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/16/2017 09:31 AM, Jan Beulich wrote:
>>>> On 16.03.17 at 13:44, <boris.ostrovsky@xxxxxxxxxx> wrote:
>> On 03/16/2017 06:40 AM, Jan Beulich wrote:
>>>>>> On 15.03.17 at 20:48, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>> --- a/xen/arch/x86/time.c
>>>> +++ b/xen/arch/x86/time.c
>>>> @@ -2051,17 +2051,11 @@ 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)
>>>> -         */
>>>> +
>>>>          if ( tsc_mode == TSC_MODE_DEFAULT && host_tsc_is_safe() &&
>>>> -             (has_hvm_container_domain(d) ?
>>>> -              (d->arch.tsc_khz == cpu_khz ||
>>>> -               hvm_get_tsc_scaling_ratio(d->arch.tsc_khz)) :
>>>> -              incarnation == 0) )
>>>> +             (d->arch.tsc_khz == cpu_khz || incarnation == 0 ||
>>> Is the incarnation comparison really needed here, i.e. doesn't it
>>> being zero imply the two frequencies to match in default mode?
>> It is not necessary but I wanted to keep it for clarity so that it is
>> explicit that when a domain is born we don't use vtsc.
> Well, considering the history here, I think its presence is rather
> going to raise questions than to answer any, so if anything I'd
> suggest to have an ASSERT() to that effect, at once serving as
> sort of documentation. But I may be the only one thinking this
> way ...


Haven't thought about an ASSERT. I can do that. Something like

ASSERT(gtsc_khz == 0 ? incarnation == 0 : true);

-boris


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