[Xen-devel] RE: Live migration fails due to c/s 20627

["Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>]

Jeremy Fitzhardinge wrote:
> On 12/15/09 08:10, Xu, Dongxiao wrote:
>>> Why not just add the code to do rdtscp emulation,
>>> which would NOT break live migration?
>>> 
>> Add rdtscp emulation has such problem that, in Intel VMX, the
>> vmexit control for rdtsc and rdtscp is the same, so if we trap
>> rdtscp for emulation, OS will suffer from looooots of rdtsc vmexit,
>> which will bring performance downgrade.
>> 
> 
> I don't see why that's relevant.  In the case where you've migrated
> the domain, if the CPU has rdtsc but not rdtscp, won't the rdtscp
> vmexit with an illegal instruction trap?  In that case you can
> emulate rdtscp while still having direct execution of rdtsc.

If CPU has rdtsc but no rdtscp, then the VM exec control bit in VMCS
won't be turned on. Therefore if rdtscp instruction runs, it will encounter
invalid op code directly but no VMEXIT.
> 
> Of course, having a wide difference between rdtscp and rdtsc
> performance may cause its own set of problems.
> 
>      J



Best Regards,
-- Dongxiao
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