Re: [Qemu-devel] [PATCH 2/3] push CPUID level to 4 to allow Intel multicore decoding

[Andre Przywara]

Avi Kivity wrote:
> On 08/19/2009 04:42 PM, Andre Przywara wrote:
> > Intel CPUs store the number of cores in CPUID leaf 4. So push
> > the maxleaf value to 4 to allow the guests access to this leaf.
> >    
>
> There's a slight compatibility risk here.  If a guest has broken 
> handling for cpuid level 4, then upgrading qemu would cause it to behave 
> differently.
>
> I don't think that's an issue for this patch, just highlighting the need 
> for a systematic treatment of backward compatibility.

If you have real headaches about it, I have two options:

What about allowing level to be specified at -cpu command line? This 
would allow users to say -cpu qemu64,level=2 if they experience 
problems. The default would stay at level 4.

The other option would be to push the level only to four if we use more 
than one thread or core.

In my research it turned out that Intel pushed the level beyond 4 with 
Pentium4 Prescott (probably with the introduction of real dual core 
chips to differentiate threads and cores), so this is quite some time 
ago. So I doubt that there are serious issues out there. The only 
problem I can think of is that the advertised cache topology is somehow 
bogus and could confuse OSes.

Regards,
Andre.

--
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
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