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

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Message-ID: <20181024192137.GI6218@tassilo.jf.intel.com>
Date:   Wed, 24 Oct 2018 12:21:37 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [v3 03/12] x86/fsgsbase/64: Add intrinsics/macros for FSGSBASE
 instructions

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@...el.com> wrote:
> >
> > From: Andi Kleen <ak@...ux.intel.com>
> >
> > 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@...nel.org> # 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