Re: [PATCH v4 1/3] x86: Use native RDTSC(P) execution when guest and host frequencies are the same

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 04/16/2014 10:37 AM, Jan Beulich wrote:
>>>> On 16.04.14 at 16:28, <boris.ostrovsky@oracle.com> wrote:
>> On 04/16/2014 07:38 AM, Jan Beulich wrote:
>>>>>> On 16.04.14 at 03:27, <boris.ostrovsky@oracle.com> wrote:
>>>> @@ -1889,10 +1890,14 @@ void tsc_set_info(struct domain *d,
>>>>            d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
>>>>            d->arch.tsc_khz = gtsc_khz ? gtsc_khz : cpu_khz;
>>>>            set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000 );
>>>> -        /* use native TSC if initial host has safe TSC, has not migrated
>>>> -         * yet and tsc_khz == cpu_khz */
>>>> -        if ( host_tsc_is_safe() && incarnation == 0 &&
>>>> -                d->arch.tsc_khz == cpu_khz )
>>>> +        /*
>>>> +         * Use native TSC if initial host has safe TSC and either has not
>>>> +         * migrated yet or tsc_khz == cpu_khz (either "naturally" or via
>>>> +         * TSC scaling)
>>>> +         */
>>>> +        if ( host_tsc_is_safe() &&
>>>> +             (incarnation == 0 || d->arch.tsc_khz == cpu_khz ||
>>>> +              cpu_has_tsc_ratio) )
>>> Doesn't this cpu_has_tsc_ratio check also need to be qualified with
>>> is_pv_domain()? And is the change from && in the old condition to ||
>>> actually valid for PV guests?
>> Hmm, I haven't thought about PV here.
>>
>> So then the condition should be
>>
>> if ( host_tsc_is_safe() )
>> {
>>       if ( (is_hvm_domain() && (arch.tsc_khz == cpu_khz || cpu_has_tsc_ratio)) ||
>>             (incarnation == 0 && d->arch.tsc_khz == cpu_khz) )
>>                 d->arch.vtsc = 0;
>> }
> Almost - to include PVH you need to either use !is_pv_domain() or
> has_hvm_container_domain().

PVH never makes here, it is forced to use TSC_MODE_NEVER_EMULATE above 
(see pvhfixme above).

PVH need to be looked at anyway. For example, there is a is_hvm_domain() 
check at the bottom which I suspect needs to be PVH-safe. I think it 
should be addressed separately though.

-boris