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

Segher Boessenkool
2020-05-05T15:33:21Z

Hi!

On Wed, May 06, 2020 at 12:27:58AM +1000, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@c-s.fr> 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@c-s.fr>
> > ---
> > 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.

[ You shouldn't use 4.6.3, there has been 4.6.4 since a while. And 4.6
is nine years old now. Most projects do not support < 4.8 anymore, on
any architecture. ]

> Plain "m" works, how much does the "<>" affect code gen in practice?
>
> A quick diff here shows no difference from removing "<>".

It will make it impossible to use update-form instructions here. That
probably does not matter much at all, in this case.

If you remove the "<>" constraints, also remove the "%Un" output modifier?

Segher