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

[Christophe Leroy]

Hi,

Le 05/05/2020 Ã 16:27, Michael Ellerman a ÃcritÂ:
> Christophe Leroy <christophe.leroy@xxxxxx> writes:
> > unsafe_put_user() is designed to take benefit of 'asm goto'.
> >
> > Instead of using the standard __put_user() approach and branch
> > based on the returned error, use 'asm goto' and make the
> > exception code branch directly to the error label. There is
> > no code anymore in the fixup section.
> >
> > This change significantly simplifies functions using
> > unsafe_put_user()
> ...
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > ---
> >   arch/powerpc/include/asm/uaccess.h | 61 +++++++++++++++++++++++++-----
> >   1 file changed, 52 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> > index 9cc9c106ae2a..9365b59495a2 100644
> > --- a/arch/powerpc/include/asm/uaccess.h
> > +++ b/arch/powerpc/include/asm/uaccess.h
> > @@ -196,6 +193,52 @@ do {								\
> >   })
> >   
> >   
> > +#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@xxxxxx/

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

Christophe