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 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.


-boris


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