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 10:32:45 -0500

Hi!

On Wed, May 06, 2020 at 12:27:58AM +1000, Michael Ellerman wrote:
> Christophe Leroy <[email-masked]> 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 <[email-masked]>
> > ---
> >  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