Re: [Xen-devel] [PATCH v3] new config option vtsc_tolerance_khz to avoid TSC emulation

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.03.18 at 12:07, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 06/03/18 10:41, Olaf Hering wrote:
>> With this option the host admin can decide how a domU should behave when
>> it is migrated across systems of the same class. Since there is always
>> some jitter when Xen calibrates the cpu_khz value, all hosts of the same
>> class will most likely have slightly different values. As a result vTSC
>> emulation is unavoidable. Data collected during the incident which
>> triggered this change showed a jitter of up to 200 KHz across systems of
>> the same class.
> 
> We should see about using better sources of information.  For one, many
> Intel systems actually expose the TSC frequency in the bottom of the
> PLATFORM_INFO MSR, although this isn't architectural, and has been
> replaced with CPUID information in Skylake.

The question is how precise this information is - I can't imagine it
is any better than the nominal clock, which then may be as far off
the actual one as the jitter Olaf is talking about.

Jan


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