RE: [Xen-devel] [PATCH] TSC scaling for live migration between platforms with different TSC frequecies

["Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>]

Ian Pratt wrote:
>>     This patchset targets for enabling TSC scaling in software for
>> live migration between platforms with different TSC frequecies. 
>> Once found the target host's frequency is different with source
>> host's, hypervisor will trap and emulate guest's all rdtsc
>>     instructions with its expected frequency. If hardware's TSC
>> frequency is difffernt with guest's exepcted freq, guest may behave
>> abnormally, eg. incorrect wallclock, soft lockup, even hang in some
>> cases.  Therefore, this patchset is necessary to avoid such issues. 
>> 
>> PATCH 0001-- Save guest's preferred TSC in image for save/restore and
>> migration PATCH 0002-- Move multidiv64 as a library function.
>> PATCH 0003-- Scaling host TSC freqeuncy patch.
> 
> I think this needs to be a feature which is enabled/disabled on a per
> VM basis (in the config file). 
> 
> I'm not sure what the default should be. Windows VMs and applications
> don't seem to much care about the TSC which is an argument for
> leaving the default as it is at the moment. However, one could argue
> that things that don't care about the TSC aren't going to be reading
> it much, so the overhead of making the default to scale the TSC
> shouldn't be too high.

Hi, Ian
   We also considered to add an option to disable/enable this feature. But you 
know, this logic is only efffective after migration, but option should be 
determined in guest creation stage, so it maybe useless for most of cases.  
Even if windows system doesn't care much about tsc, but we can't say 
applications don't use it at all. I assume Windows's timing mechanism also need 
build the relationship between tsc and time source, and if the assumption is 
right, guest's applications may behave abnoramlly in a different tsc frequency. 
 Thanks! :-)
Xiantao
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