linux-kernel - Re: [PATCH v2 1/3] powerpc/uaccess: Don't use "m<>" constraint with GCC 4.9

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Date:   Sat, 24 Oct 2020 21:27:57 +1100 (AEDT)
From:   Michael Ellerman <patch-notifications@...erman.id.au>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        mathieu.desnoyers@...icios.com
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] powerpc/uaccess: Don't use "m<>" constraint with GCC 4.9

On Tue, 20 Oct 2020 07:40:07 +0000 (UTC), Christophe Leroy wrote:
> GCC 4.9 sometimes fails to build with "m<>" constraint in
> inline assembly.
> 
>   CC      lib/iov_iter.o
> In file included from ./arch/powerpc/include/asm/cmpxchg.h:6:0,
>                  from ./arch/powerpc/include/asm/atomic.h:11,
>                  from ./include/linux/atomic.h:7,
>                  from ./include/linux/crypto.h:15,
>                  from ./include/crypto/hash.h:11,
>                  from lib/iov_iter.c:2:
> lib/iov_iter.c: In function 'iovec_from_user.part.30':
> ./arch/powerpc/include/asm/uaccess.h:287:2: error: 'asm' operand has impossible constraints
>   __asm__ __volatile__(    \
>   ^
> ./include/linux/compiler.h:78:42: note: in definition of macro 'unlikely'
>  # define unlikely(x) __builtin_expect(!!(x), 0)
>                                           ^
> ./arch/powerpc/include/asm/uaccess.h:583:34: note: in expansion of macro 'unsafe_op_wrap'
>  #define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)
>                                   ^
> ./arch/powerpc/include/asm/uaccess.h:329:10: note: in expansion of macro '__get_user_asm'
>   case 4: __get_user_asm(x, (u32 __user *)ptr, retval, "lwz"); break; \
>           ^
> ./arch/powerpc/include/asm/uaccess.h:363:3: note: in expansion of macro '__get_user_size_allowed'
>    __get_user_size_allowed(__gu_val, __gu_addr, __gu_size, __gu_err); \
>    ^
> ./arch/powerpc/include/asm/uaccess.h:100:2: note: in expansion of macro '__get_user_nocheck'
>   __get_user_nocheck((x), (ptr), sizeof(*(ptr)), false)
>   ^
> ./arch/powerpc/include/asm/uaccess.h:583:49: note: in expansion of macro '__get_user_allowed'
>  #define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)
>                                                  ^
> lib/iov_iter.c:1663:3: note: in expansion of macro 'unsafe_get_user'
>    unsafe_get_user(len, &uiov[i].iov_len, uaccess_end);
>    ^
> make[1]: *** [scripts/Makefile.build:283: lib/iov_iter.o] Error 1
> 
> [...]

Patch 1 applied to powerpc/fixes.

[1/3] powerpc/uaccess: Don't use "m<>" constraint with GCC 4.9
      https://git.kernel.org/powerpc/c/592bbe9c505d9a0ef69260f8c8263df47da2698e

cheers