A little bit suspicion for the usage model too :(
The CPUID feature list can't be changed dynamically, so I doutb
if a real user will give up performance at beginning when he/she
starts the guest for "maybe" future migration cross host platform.

A VT or SVM enabled machine usually has many features 
which doesn't exist in the common feature list for both Intel & AMD,
such as MMX/SSE2/SSE3. So the performance penalty could be big.

Eddie
 


________________________________

        From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Dor Laor
        Sent: 2007年12月16日 20:50
        To: Joerg Roedel
        Cc: kvm-devel; Avi Kivity
        Subject: Re: [kvm-devel] question on #UD emulation
        
        
        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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