Re: [PATCH] x86: use builtins to read eflags

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, Dec 16, 2021 at 12:00 PM Bill Wendling <morbo@google.com> wrote:
>
> ()On Wed, Dec 15, 2021 at 3:26 PM Peter Zijlstra <peterz@infradead.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