Re: question on #UD emulation

[Dor Laor <dor.laor-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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Joerg Roedel wrote:
>
> On Sun, Dec 16, 2007 at 01:32:40PM +0200, Avi Kivity wrote:
> > Dong, Eddie wrote:
> >
> > > BTW, do we need to support live migrating an AMD VM to Intel 
> platform or
> > > vice versa
> >
> > Yes, it allows users to buy the machines with the best cost/performance
> > ratio, rather than continue to buy machines from the same vendor all the
> > time.
>
> I don't agree with this. A user can still buy machines from both vendors
> and migrate offline it he wants to. In general I think that allowing
> live migration between Intel and AMD hosts is not a good idea because
> you limit the guest to the subset of the CPU features available on both
> platforms. This disallows many optimizations and costs performance for
> the guest.
>
Having the option to live migrate between various hosts is a powerful 
feature.
Of course it costs some performance penalty but it might not be relevant 
for the user
work load.
By having cmd line parameters to control the cpuid and cpu type one can 
dynamically calculate
the lowest common denominator and to run his virtual machines using it.
One can always pass -cpu=host to have maximum performance.
For users who like to live migrate between Amd-Intel we'll use one of 
cpu types
for both.
Regards,
Dor
>
> Take the problem Dor mentioned this week about the performance impact of
> the gettimeofday() syscall causing many cpuid guest exits. If we had
> not to support migration between AMD and Intel we could simply propagate
> FEATURE_RDTSCP to an guest on AMD an SYNC_RDTSC on Intel platforms. The
> Linux get_cycles_sync() function would then no longer execute CPUID.
> This is only one example, I found some more during my work on KVM.
>
> Joerg
>
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>
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