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

[Jim Mattson]

On Mon, Mar 2, 2020 at 10:02 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 02/03/20 18:44, Jim Mattson wrote:
> > 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.
>
> Yeah, I think GET_CPUID2 with the same number of leaves that you have
> passed to SET_CPUID2 should work.

Having said that, it doesn't look like the method that invokes this
ioctl (in Google's code base) gets called from anywhere.