Re: [Xen-devel] PV performance degraded after live migration

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.03.17 at 16:26, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 15/03/17 15:23, Olaf Hering wrote:
>> On Wed, Mar 15, Olaf Hering wrote:
>>
>>> On Wed, Mar 15, Andrew Cooper wrote:
>>>> As a crazy idea, doest this help?
>>>> tsc->incarnation = 0
>> This does indeed help. One system shows now the results below, which
>> means the performance goes down during migration (to localhost) and goes
>> back to normal after migration.
>>
>> What impact has such change to ->incarnation?
> 
> So what this means is that, after migrate, Xen sees that the advertised
> TSC value doesn't match the current hardwares TSC, so enables rdtsc
> interception so the values reported back can be emulated at the old
> frequency.
> 
> There is no easy solution to this problem.

Especially for localhost migration it ought to be possible to deal
with this. What I'm wondering is why the frequency comparison
in default mode handling in tsc_set_info() is HVM-only:

        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) )
        {

Boris, you've made it be this way in 82713ec8d2 ("x86: use native
RDTSC(P) execution when guest and host frequencies are the
same").

Jan


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