Re: [PATCH v2] KVM: X86: deprecate obsolete KVM_GET_CPUID2 ioctl

[Jim Mattson]

On Mon, Mar 2, 2020 at 9:09 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 02/03/20 18:01, Jim Mattson wrote:
> >> And in fact, it's not used anywhere. So it should be
> >> deprecated.
> > I don't know how you can make the assertion that this ioctl is not
> > used anywhere. For instance, I see a use of it in Google's code base.
>
> Right, it does not seem to be used anywhere according to e.g. Debian
> code search but of course it can have users.
>
> What are you using it for?  It's true that cpuid->nent is never written
> back to userspace, so the ioctl is basically unusable unless you already
> know how many entries are written.  Or unless you fill the CPUID entries
> with garbage before calling it, I guess; is that what you are doing?

One could use GET_CPUID2 after SET_CPUID2, to see what changes kvm
made to the requested guest CPUID information without telling you.