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

From: Segher Boessenkool <[email-masked]>
Date: Wed, 6 May 2020 12:58:49 -0500

On Wed, May 06, 2020 at 10:58:55AM +1000, Michael Ellerman wrote:
> >> 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.  ]
> 
> Moving up to 4.6.4 wouldn't actually help with this though would it?

Nope.  But 4.6.4 is a bug-fix release, 91 bugs fixed since 4.6.3, so you
should switch to it if you can :-)

> Also I have 4.6.3 compilers already built, I don't really have time to
> rebuild them for 4.6.4.
> 
> The kernel has a top-level minimum version, which I'm not in charge of, see:
> 
> https://www.kernel.org/doc/html/latest/process/changes.html?highlight=gcc

Yes, I know.  And it is much preferred not to have stricter requirements
for Power, I know that too.  Something has to give though :-/

> There were discussions about making 4.8 the minimum, but I'm not sure
> where they got to.

Yeah, just petered out I think?

All significant distros come with a 4.8 as system compiler.

> >> 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?
> 
> So like this?
> 
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index 62cc8d7640ec..ca847aed8e45 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -207,10 +207,10 @@ do {								\
>  
>  #define __put_user_asm_goto(x, addr, label, op)			\
>  	asm volatile goto(					\
> -		"1:	" op "%U1%X1 %0,%1	# put_user\n"	\
> +		"1:	" op "%X1 %0,%1	# put_user\n"		\
>  		EX_TABLE(1b, %l2)				\
>  		:						\
> -		: "r" (x), "m<>" (*addr)				\
> +		: "r" (x), "m" (*addr)				\
>  		:						\
>  		: label)

Like that.  But you will have to do that to *all* places we use the "<>"
constraints, or wait for more stuff to fail?  And, there probably are
places we *do* want update form insns used (they do help in some loops,
for example)?

Segher