Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'

Segher Boessenkool
2020-05-05T16:00:21Z

On Tue, May 05, 2020 at 05:40:21PM +0200, Christophe Leroy wrote:
> >>+#define __put_user_asm_goto(x, addr, label, op) \
> >>+ asm volatile goto( \
> >>+ "1: " op "%U1%X1 %0,%1 # put_user\n" \
> >>+ EX_TABLE(1b, %l2) \
> >>+ : \
> >>+ : "r" (x), "m<>" (*addr) \
> >
> >The "m<>" here is breaking GCC 4.6.3, which we allegedly still support.
> >
> >Plain "m" works, how much does the "<>" affect code gen in practice?
> >
> >A quick diff here shows no difference from removing "<>".
>
> It was recommended by Segher, there has been some discussion about it on
> v1 of this patch, see
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/4fdc2aba6f5e51887d1cd0fee94be0989eada2cd.1586942312.git.christophe.leroy@c-s.fr/
>
> As far as I understood that's mandatory on recent gcc to get the
> pre-update form of the instruction. With older versions "m" was doing
> the same, but not anymore.

Yes. How much that matters depends on the asm. On older CPUs (6xx/7xx,
say) the update form was just as fast as the non-update form. On newer
or bigger CPUs it is usually executed just the same as an add followed
by the memory access, so it just saves a bit of code size.

> Should we ifdef the "m<>" or "m" based on GCC
> version ?

That will be a lot of churn. Just make 4.8 minimum?

Segher