Re: [Qemu-devel] cpuid problem in upstream qemu with kvm

[Avi Kivity]

On 12/14/2009 10:18 PM, Anthony Liguori wrote:
> Michael S. Tsirkin wrote:
> > This might help 32 bit guests, but not guests with 64 bit
> > kernel and 32 bit userspace (my case) because all 64 bit
> > CPUs advertise syscall bit in cpuid. Thus 64 bit guests
> > do not seem to even bother checking this bit:
> > AMD + 64 bit -> syscall.
>
> Okay, I don't see a great option other than migrating the vendor_id 
> string.

That's not strictly necessary since the guest cannot change the vendor 
string; all the user has to do is to launch both guests with explicit 
vendor ids.  Of course that imposes more on the user (or the management 
application).

> Otherwise, cross vendor migration will not work by default.  Maybe 
> that's not a problem but if so, we really should change the default 
> cpu model to be much more aggressive about exposing host features.

Maybe we should make -cpu host the default.  That will give the best 
performance for casual users, more testing for newer features, and will 
force management apps to treat migration much more seriously.  The 
downside is that casual users upgrading their machines might experience 
issues with Windows.  Feature compatibility is not just about migration.

--
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