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


From: Avi Kivity
Subject: Re: [Qemu-devel] Re: [kvm-devel] expose host CPU features to guests
Date: Thu, 06 Sep 2007 11:46:07 +0300
User-agent: Thunderbird 2.0.0.5 (X11/20070719)

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.





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