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

From: Segher Boessenkool <[email-masked]>
Date: Tue, 5 May 2020 10:59:44 -0500

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/4fdc2aba[email-masked]/
> 
> 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