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

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>]

>     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.

Ian



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