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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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.
Of course, having a wide difference between rdtscp and rdtsc performance 
may cause its own set of problems.
    J

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