Re: [PATCH] x86/asm: Replace __force_order with memory clobber

[Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>]

Hi Arvind,

On Sun, Aug 23, 2020 at 11:25 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> - Using a dummy input operand with an arbitrary constant address for the
>   read functions, instead of a global variable. This will prevent reads
>   from being reordered across writes, while allowing memory loads to be
>   cached/reordered across CRn reads, which should be safe.

Assuming no surprises from compilers, this looks better than dealing
with different code for each compiler.

> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602

A lore link to the other discussion would be nice here for context.

> + * The compiler should not reorder volatile asm, however older versions of GCC
> + * had a bug (which was fixed in 8.1, 7.3 and 6.5) where they could sometimes

I'd mention the state of GCC 5 here.

> + * reorder volatile asm. The write functions are not a problem since they have
> + * memory clobbers preventing reordering. To prevent reads from being reordered
> + * with respect to writes, use a dummy memory operand.
>   */
> -extern unsigned long __force_order;
> +

Spurious newline?

Cheers,
Miguel