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

[Avi Kivity <avi@qumranet.com>]

Paul Brook wrote:
>>> I think qemu-cvs has a -cpu option for non-x86 which could be used for
>>> this.  Agree machine types are the wrong approach.
>>>       
>> Yep, machine types are already used to switch between a different concept
>> so using  the new -cpu option would make sense. Could perhaps extend the
>> syntax so that instead of '-cpu TYPE' it used '-cpu TYPE,FEATURES' where
>> FEATURES was an optional list of CPU features to allow
>>     
>
> I tried this for ARM, and having separate type+features isn't worth the 
> effort. The internal implementation is feature based, but IMHO there's little 
> benefit exposing that to the user. Just define appropriate CPUs for the 
> interesting feature combinations.
>
>   

The use case is different.  We don't care about the actual features, but 
about finding the greatest common denominator in a virtualization farm.  
I don't see this as useful for qemu; rather kvm and kqemu.

> Of course the x86 emulation doesn't currently support restricting the 
> architecture features available.  To make the --cpu option useful you need to 
> implement that first.
>   

Applications will not use a feature that is not present in cpuid, so 
that is not an issue.  For the virtualization use case, it is also 
impossible to turn off support for a feature.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.