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

[Michael S. Tsirkin]

On Sun, Dec 20, 2009 at 11:49:40AM +0200, Avi Kivity wrote:
> 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.

This seems very aggressive.  Can't we whitelist features that we know
about?  Further, doesn't KVM already do this?

-- 
MST