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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Dec 15, 2021 at 01:18:47PM -0800, Bill Wendling wrote:
> GCC and Clang both have builtins to read from and write to the
> EFLAGS register. This allows the compiler to determine the best way
> to generate the code, which can improve code generation.

Only because clang is still brain-dead wrt "rm" constraints, right?

> Signed-off-by: Bill Wendling <morbo@google.com>
> ---
>  arch/x86/include/asm/irqflags.h | 24 +++++-------------------
>  1 file changed, 5 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
> index c5ce9845c999..574fb44b82f7 100644
> --- a/arch/x86/include/asm/irqflags.h
> +++ b/arch/x86/include/asm/irqflags.h
> @@ -15,25 +15,11 @@
>   * Interrupt control:
>   */
>  
> -/* Declaration required for gcc < 4.9 to prevent -Werror=missing-prototypes */
> -extern inline unsigned long native_save_fl(void);
> -extern __always_inline unsigned long native_save_fl(void)
> -{
> -	unsigned long flags;
> -
> -	/*
> -	 * "=rm" is safe here, because "pop" adjusts the stack before
> -	 * it evaluates its effective address -- this is part of the
> -	 * documented behavior of the "pop" instruction.
> -	 */
> -	asm volatile("# __raw_save_flags\n\t"
> -		     "pushf ; pop %0"
> -		     : "=rm" (flags)
> -		     : /* no input */
> -		     : "memory");
> -
> -	return flags;
> -}
> +#ifdef CONFIG_X86_64
> +#define native_save_fl()	__builtin_ia32_readeflags_u64()
> +#else
> +#define native_save_fl()	__builtin_ia32_readeflags_u32()
> +#endif

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.

I'm thinking you wrecked that bit.