linux-kernel - Re: [PATCH] s390/jump_label: Correct asm contraint

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Date:   Wed, 23 Jan 2019 14:24:01 +0100
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     Laura Abbott <labbott@...hat.com>,
        Andreas Krebbel <krebbel@...ux.ibm.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] s390/jump_label: Correct asm contraint

On Wed, Jan 23, 2019 at 01:55:13PM +0100, Laura Abbott wrote:
> There's a build failure with gcc9:
> 
>  ./arch/s390/include/asm/jump_label.h: Assembler messages:
>  ./arch/s390/include/asm/jump_label.h:23: Error: bad expression
>  ./arch/s390/include/asm/jump_label.h:23: Error: junk at end of line, first unrecognized character is `r'
>  make[1]: *** [scripts/Makefile.build:277: init/main.o] Error 1
> 
> According to the toolchain people, the actual issue is the use of
> "X" constraint which is too permissive. Switch to using "i" instead.
> 
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1668703
> Signed-off-by: Laura Abbott <labbott@...hat.com>
> ---
>  arch/s390/include/asm/jump_label.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/include/asm/jump_label.h b/arch/s390/include/asm/jump_label.h
> index e2d3e6c43395..41dabfd8518d 100644
> --- a/arch/s390/include/asm/jump_label.h
> +++ b/arch/s390/include/asm/jump_label.h
> @@ -22,7 +22,7 @@ static inline bool arch_static_branch(struct static_key *key, bool branch)
>  			  ".long	0b-.,%l[label]-.\n"
>  			  ".quad	%0-.\n"
>  			  ".popsection\n"
> -			  : : "X" (&((char *)key)[branch]) : : label);
> +			  : : "i" (&((char *)key)[branch]) : : label);
>  	return false;
>  label:
>  	return true;

Hmmm, this works only for the kernel image, but not for modules, which
we compile with "-fPIC", which again doesn't work as described in the
referenced bugzilla:

In file included from ././include/linux/compiler_types.h:68,
                 from <command-line>:
./arch/s390/include/asm/jump_label.h: In function 'kvm_vcpu_ioctl':
./include/linux/compiler-gcc.h:124:38: warning: asm operand 0 probably doesn't match constraints
 #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)
                                      ^~~
./arch/s390/include/asm/jump_label.h:19:2: note: in expansion of macro 'asm_volatile_goto'
  asm_volatile_goto("0: brcl 0,"__stringify(JUMP_LABEL_NOP_OFFSET)"\n"
  ^~~~~~~~~~~~~~~~~
./include/linux/compiler-gcc.h:124:38: error: impossible constraint in 'asm'
 #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)
                                      ^~~
./arch/s390/include/asm/jump_label.h:19:2: note: in expansion of macro 'asm_volatile_goto'
  asm_volatile_goto("0: brcl 0,"__stringify(JUMP_LABEL_NOP_OFFSET)"\n"

Andreas, Ilya, any idea how to fix this?