Re: [Xen-devel] [PATCH] x86/viridian: Add Partition Reference Time enlightenment

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.09.14 at 18:18, <Paul.Durrant@xxxxxxxxxx> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 25 September 2014 17:11
>> To: Paul Durrant
>> Cc: Christoph Egger; Ian Campbell; Ian Jackson; Stefano Stabellini; xen-
>> devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org)
>> Subject: Re: [PATCH] x86/viridian: Add Partition Reference Time
>> enlightenment
>> 
>> >>> On 25.09.14 at 17:50, <paul.durrant@xxxxxxxxxx> wrote:
>> > The presence of the partition reference time enlightenment persuades
>> newer
>> > versions of Windows to prefer the TSC as their primary time source. Hence,
>> > if rdtsc is not being emulated and is invariant then many vmexits (for
>> > alternative time sources such as the HPET or reference counter MSR) can
>> > be avoided.
>> >
>> > If the VM is migrated to a host where TSC is no longer invariant and/or
>> > TSC is emulated then the enlightenment is disabled.
>> 
>> Which means performance of a guest post migration can change
>> dramatically. Is that really a good thing?
> 
> I guess that's debatable. It is possible to use this enlightenment to scale 
> TSC, which means  that TSC emulation could be avoided even if the tsc_khz 
> changes, but I still need to figure out how best to do that part. Nothing can 
> be done in the case of migration to a non iTSC host though.
> I could leave this out the default enlightenment set for now.

Yes please, perhaps with a brief code comment as to why.

Jan


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