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

Avi Kivity
2007-09-06T08:46:07Z

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.