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

[Avi Kivity <avi@redhat.com>]

On 08/20/2009 01:36 PM, Andre Przywara wrote:
> 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:

It's really an imaginary headache.

>
> 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.

I think this is the best option.

> 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. 

I only pointed this out as an example of a new feature that has an 
effect even if it is not used, something we should beware of.

> The only problem I can think of is that the advertised cache topology 
> is somehow bogus and could confuse OSes.

So long as it's smaller than contemporary caches we should be fine.

btw, does -cpu host use the host cpu cache information?

-- 
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