Re: [PATCH 0/2] KVM: x86: Propagate AMD-specific IBRS bits to guests

[Paolo Bonzini]

On 3/6/23 22:44, Borislav Petkov wrote:
> > I thought that the consensus was that adding unused-by-the-kernel flags to
> > cpufeatures.h is ok so long as the feature is hidden from /proc/cpuinfo and the
> > kernel already dedicates a word to the CPUID leaf?
> I guess we should finally write it down in Documentation/x86/cpuinfo.rst
>
> And in case there's no dedicated word, it should be resorted to KVM-only
> feature flags.
>
> In any case, I'd like for baremetal CPUID stuff to be decoupled from
> KVM's machinery as far as possible as both have different goals wrt
> feature flags.

It's very rare that KVM can provide a CPUID feature if the kernel has 
masked it, so if the kernel needs to know about a feature word than KVM 
most likely needs to know what kind of massaging the kernel has done.

Paolo