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

Roedel, Joerg
2011-06-11T10:39:57Z

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. 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.

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.

Regards,

Joerg

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