linux-kernel - Re: [PATCH] x86: use builtins to read eflags

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Date:   Thu, 16 Dec 2021 12:07:16 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Bill Wendling <morbo@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>, llvm@...ts.linux.dev,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: use builtins to read eflags

On Thu, Dec 16, 2021 at 12:00 PM Bill Wendling <morbo@...gle.com> wrote:
>
> ()On Wed, Dec 15, 2021 at 3:26 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Also note the thing was extern inline, and there's actually an
> > out-of-line symbol for them too. The out-of-line thing is explicitly
> > using %rax due to paravirt muck.

Oh, shoot, I wrote that.
d0a8d9378d16e
That was before we had __attribute__(no_stack_protector). Once the
minimal supported versions of both compilers support that, I'd love to
revert d0a8d9378d16e.

extern inline is the thing that makes me most nervous about moving the
kernel from -std=gnu89 to anything newer.  I should check whether
gnu99 uses c99 extern inline or gnu inline...

> >
> > I'm thinking you wrecked that bit.
>
> If you prefer, it could be written like so:
>
> extern inline unsigned long native_save_fl(void);
> extern __always_inline unsigned long native_save_fl(void)
> {
> #ifdef CONFIG_X86_64
>     return __builtin_ia32_readeflags_u64();
> #else
>     return __builtin_ia32_readeflags_u32();
> #endif
> }

Yes, that would fix the `extern inline` issue. I wonder if this is prettier as:

return IS_ENABLED(CONFIG_X86_64) ? __builtin_ia32_readeflags_u64() :
__builtin_ia32_readeflags_u32();
-- 
Thanks,
~Nick Desaulniers