Re: [v3 03/12] x86/fsgsbase/64: Add intrinsics/macros for FSGSBASE instructions

[Andi Kleen]

On Wed, Oct 24, 2018 at 11:53:54AM -0700, Andy Lutomirski wrote:
> On Tue, Oct 23, 2018 at 11:43 AM Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
> >
> > From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> >
> > Add C intrinsics and assembler macros for the new FSBASE and GSBASE
> > instructions.
> >
> > Very straight forward. Used in followon patches.
> >
> > [ luto: Rename the variables from FS and GS to FSBASE and GSBASE and
> >   make <asm/fsgsbase.h> safe to include on 32-bit kernels. ]
> >
> > v2: Use __always_inline
> >
> > [ chang: Revise the changelog. Place them in <asm/fsgsbase.h>. Replace
> >   the macros with GAS-compatible ones. ]
> >
> > If GCC supports it, we can add -mfsgsbase to CFLAGS and use the builtins
> > here for extra performance.
> 
> Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx> # C parts only
> 
> With the caveat that I'm not convinced that the memory clobbers are
> needed.  The __force_order trick in special_insns.h would probably be
> more appropriate.
> 
> I don't feel qualified to review the asm part without some research.
> Whereas hpa or Boris could probably review it with their eyes closed.

BTW the other option would be to update the min-binutils requirement 
to 2.21 (currently it is 2.20) and then write it directly without .byte. 
I believe 2.21 added support for these instructions.

(It's only a binutils requirement, don't need gcc support)

-Andi