Re: semantics of "-cpu host" and "check"/"enforce"

[Avi Kivity <avi@redhat.com>]

On 06/11/2011 01:40 PM, Roedel, Joerg wrote:
> On Fri, Jun 10, 2011 at 05:36:37PM -0400, Eduardo Habkost wrote:
>
> >  We have 3 sets of cpu features that may or may not be included in
> >  '-cpu host':
> >
> >  A) Features that are supported by the host and that KVM can already
> >     emulate, or don't need KVM support to be used;
> >  B) Features that may be not supported by the host but can be emulated by
> >     KVM (e.g. the SVM features, or x2apic);
> >  C) Features that are supported by the host but KVM can't emulate.
> >     Divided in:
> >     C1) features we can't emulate and we know about it (e.g. dtes64)[1]
> >     C2) features we possibly can't emulate but we don't even know about it
> >         (e.g. features added to recent CPUs).
> >
> >  It seems obvious that all the features in group A must always be
> >  included in '-cpu host', but what about features in the B or C groups?
> >
> >
> >  About group B: it looks like we are not being consistent. For example,
> >  svm_features has every bit enabled when using '-cpu host' even if the
> >  host doesn't support them; in other cases (e.g. x2apic), it is not
> >  enabled by '-cpu host' unless the host already supports it.
>
> SVM is a special feature. We can't just forward the host-cpuid to the
> guest because every SVM feature we provide to the guest needs emulation
> in the kernel module. The kernel-module will tell qemu-kvm about its
> feature set via the GET_SUPPORTED_CPUID ioctl.

SVM isn't special in this regard.  It's potentially true for any 
feature, and actually true for some of them.

> So the idea behint -cpu
> host and SVM features is that qemu-kvm enables all of them and masks out
> everything that is not supported by the kernel module.

Right (but by whitelisting known features, not blacklisting).

> Note that the kernel might even emulate features that are not supported
> on the host, like the vmcb-clean-bits, so we really need to ask the
> kernel what is supported for the guest.

x2apic is another example.

-- 
error compiling committee.c: too many arguments to function