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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 25 Oct 2018 16:14:15 -0700
Message-ID: <CALCETrUmD0jAzVMAUJ=9Vpih56Q4t9U5DJLnUxQdeMwZ3RAECA@mail.gmail.com>
Subject: Re: [v3 03/12] x86/fsgsbase/64: Add intrinsics/macros for FSGSBASE instructions
To:     Andi Kleen <ak@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Metzger, Markus T" <markus.t.metzger@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Oct 24, 2018 at 12:21 PM Andi Kleen <ak@linux.intel.com> wrote:
>
> 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@intel.com> wrote:
> > >
> > > From: Andi Kleen <ak@linux.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@kernel.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)
>

I'd personally be fine with this.  Linus? Thomas? Ingo?