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

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

Keir Fraser wrote:
> Stuffing the guest freq in a save-image pad field is not backward
> compatible. Old images will not have that field filled in and you'll
> probably end up doing something stupid like give them a zero-hertz
> TSC. Please think about backward compatibility and use a separate

Hi, Keir
    I also checked the filed to solve the backward compatibility issue, and 
once found the field is zero, we won't do anything about TSC scaling(reference 
hvm_gtsc_need_scale to get the detail), so guest never uses a zero-hertz 
frequency in any case.  You know, since old images can't provide TSC frequency 
info, so TSC scaling logic shouldn't cover it.  
Xiantao

>  -- Keir
> 
> On 22/06/2009 06:14, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx> wrote:
> 
>> Hi, Keir
>> This is the new version which has addressed the comments from the
>> mailing list. Please review it again.  Thanks!
>> Xiantao
>> 
>> Zhang, Xiantao wrote:
>>> Hi, Keir
>>> 
>>>     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.
>>> 
>>> Signed-off-by Xiantao Zhang <xiantao.zhang@xxxxxxxxx>
>>> Xiantao


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