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

[Arvind Sankar]

On Mon, Aug 24, 2020 at 09:13:34PM +0200, Miguel Ojeda wrote:
> Hi Arvind,
> 
> On Sun, Aug 23, 2020 at 11:25 PM Arvind Sankar <nivedita@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602
> 
> A lore link to the other discussion would be nice here for context.
> 

Ok.

> > + * 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.
> 

Ok.

> > + * 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?
> 

This was intentional, but I can remove it if people don't like the extra
whitespace.

I'll wait a few days for additional review comments before sending v2.

Thanks.