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

Christophe Leroy
2020-05-06T18:11:13Z

Le 06/05/2020 à 19:58, Segher Boessenkool a écrit :
> 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