Re: [kvm-devel] [Qemu-devel] expose host CPU features to guests: Take 3

[Fabrice Bellard <fabrice@bellard.org>]

Avi Kivity wrote:
> J. Mayer wrote:
> 
>>On Tue, 2007-09-25 at 12:40 +0200, Avi Kivity wrote:
>>  
>>
>>>Avi Kivity wrote:
>>>    
>>>
>>>>>>    
>>>>>>      
>>>>>>          
>>>>>
>>>>>I've got a remark about this: why this has to be added to the Qemu
>>>>>code ?
>>>>>Imho, all is needed is an implementation of the -cpu option for
>>>>>x86/x86_64 target. Then, an external tool (even a shell script) can be
>>>>>used to determine what is the host CPU if you want to select the exact
>>>>>same CPU to be emulated in Qemu. It seems to me that trying to do so is
>>>>>out of the scope of Qemu code and just add unneeded complexity.
>>>>>  
>>>>>    
>>>>>        
>>>>
>>>>Indeed for regular qemu this is useless.  But it is useful for kqemu
>>>>(for which there is support in mainline qemu), and for kvm (which we
>>>>hope to merge one day).
>>>>
>>>>  
>>>>      
>>>
>>>Actually (as Izik Eidus reminds me), this isn't very useful for kqemu as
>>>it can't trap cpuid in all circumstances.
>>>
>>>So this is mainly useful for kvm.  I hope it will be applied regardless
>>>of that, as there is agreement that kvm support should be merged.  I'd
>>>much rather pull the feature from qemu rather than carry it as an
>>>additional patch.
>>>    
>>
>>Still I don't understand why it's usefull to put it inside the emulator
>>and why:
>># qemu -cpu `guess_host_cpu`
>>would not do the work properly. Adding a specific case for '-cpu host'
>>seems useless to me.
>>And this way of doing potentially work for any family of emulated
>>targets, without any modification in Qemu. If the string returned by
>>'guess_host_cpu' is not recognized for the specific target you used it
>>with, Qemu just stops telling it cannot find this CPU model, no more, no
>>less.
>>  
> 
> 
> It's a usability issue.  I agree your suggestion would work, but I'd
> like the default for kvm to be using the host cpu features, whereas
> managed environments would specify some subset to enable live migration.
> 
> 
>>The only case it could be interresting, imho, is if you do not allow the
>>-cpu option in KVM case and force the cpu model instead using this
>>function. This behavior does not seem to be great idea to me.
>>  
> 
> 
> I think we can move the host cpu checks to kvm-specific code, since it
> is not useful for kqemu.
> 
> So, running qemu without any parameters would use host capabilities if
> kvm is available and the default qemu cpu if not.  The -cpu option can
> be used to override this if necessary.

Rectification: this is useful for kqemu too. I strongly suggest to look 
at kqemu.c:kqemu_update_cpuid() !

Moreover I believe that using the same CPU as host can be useful for 
pure emulation too, for example if code to do cache profiling is added.

Regards,

Fabrice.