Re: [PATCH 17/21] x86/fpu: Use proper mask to replace full instruction mask

[Liang, Kan]

On 6/19/2020 3:31 PM, Peter Zijlstra wrote:
> On Fri, Jun 19, 2020 at 07:04:05AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
>
> > KVM includes the header file fpu/internal.h. To avoid 'undefined
> > xfeatures_mask_all' compiling issue, xfeatures_mask_all has to be
> > exported.
>
> > diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> > index 587e03f..eb2e44e 100644
> > --- a/arch/x86/kernel/fpu/xstate.c
> > +++ b/arch/x86/kernel/fpu/xstate.c
> > @@ -58,6 +58,7 @@ static short xsave_cpuid_features[] __initdata = {
> >    * XSAVE buffer, both supervisor and user xstates.
> >    */
> >   u64 xfeatures_mask_all __read_mostly;
> > +EXPORT_SYMBOL_GPL(xfeatures_mask_all);
>
> *groan*...
>
> AFAICT KVM doesn't actually use any of those functions,

It seems KVM may eventually invoke copy_xregs_to_kernel() as below.

kvm_save_current_fpu()
    copy_fpregs_to_fpstate()
        copy_xregs_to_kernel()

I think we have to export the xfeatures_mask_all.

Thanks,
Kan

> can't we have
> something like BUILD_KVM (like BUILD_VDSO) and exclude those functions
> from the KVM build?
>
> I so detest exporting random crap because kvm..