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

[Christophe Leroy]

Le 06/05/2020 Ã 19:58, Segher Boessenkool a ÃcritÂ:
> 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)?

AFAICT, git grep "m<>" provides no result.

However many places have %Ux:

arch/powerpc/boot/io.h:	__asm__ __volatile__("lbz%U1%X1 %0,%1; twi 
0,%0,0; isync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("stb%U0%X0 %1,%0; sync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("lhz%U1%X1 %0,%1; twi 
0,%0,0; isync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("sth%U0%X0 %1,%0; sync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("lwz%U1%X1 %0,%1; twi 
0,%0,0; isync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("stw%U0%X0 %1,%0; sync"
arch/powerpc/include/asm/atomic.h:	__asm__ __volatile__("lwz%U1%X1 
%0,%1" : "=r"(t) : "m"(v->counter));
arch/powerpc/include/asm/atomic.h:	__asm__ __volatile__("stw%U0%X0 
%1,%0" : "=m"(v->counter) : "r"(i));
arch/powerpc/include/asm/atomic.h:	__asm__ __volatile__("ld%U1%X1 %0,%1" 
: "=r"(t) : "m"(v->counter));
arch/powerpc/include/asm/atomic.h:	__asm__ __volatile__("std%U0%X0 
%1,%0" : "=m"(v->counter) : "r"(i));
arch/powerpc/include/asm/book3s/32/pgtable.h:		stw%U0%X0 %2,%0\n\
arch/powerpc/include/asm/book3s/32/pgtable.h:		stw%U0%X0 %L2,%1"
arch/powerpc/include/asm/io.h:	__asm__ __volatile__("sync;"#insn"%U1%X1 
%0,%1;twi 0,%0,0;isync"\
arch/powerpc/include/asm/io.h:	__asm__ __volatile__("sync;"#insn"%U0%X0 
%1,%0"			\
arch/powerpc/include/asm/nohash/pgtable.h:			stw%U0%X0 %2,%0\n\
arch/powerpc/include/asm/nohash/pgtable.h:			stw%U0%X0 %L2,%1"
arch/powerpc/kvm/powerpc.c:	asm ("lfs%U1%X1 0,%1; stfd%U0%X0 0,%0" : 
"=m" (fprd) : "m" (fprs)
arch/powerpc/kvm/powerpc.c:	asm ("lfd%U1%X1 0,%1; stfs%U0%X0 0,%0" : 
"=m" (fprs) : "m" (fprd)


Christophe