linux-kernel - Re: [PATCH] x86: bitops: fix build regression

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Message-ID: <6A99766A-59FB-42DF-9350-80EA671A42B0@zytor.com>
Date:   Thu, 07 May 2020 00:02:32 -0700
From:   hpa@...or.com
To:     Brian Gerst <brgerst@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sedat Dilek <sedat.dilek@...il.com>,
        stable <stable@...r.kernel.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        "kernelci . org bot" <bot@...nelci.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Ilie Halip <ilie.halip@...il.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Daniel Axtens <dja@...ens.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86: bitops: fix build regression

On May 6, 2020 11:18:09 PM PDT, Brian Gerst <brgerst@...il.com> wrote:
>On Tue, May 5, 2020 at 1:47 PM Nick Desaulniers
><ndesaulniers@...gle.com> wrote:
>>
>> From: Sedat Dilek <sedat.dilek@...il.com>
>>
>> It turns out that if your config tickles __builtin_constant_p via
>> differences in choices to inline or not, this now produces invalid
>> assembly:
>>
>> $ cat foo.c
>> long a(long b, long c) {
>>   asm("orb\t%1, %0" : "+q"(c): "r"(b));
>>   return c;
>> }
>> $ gcc foo.c
>> foo.c: Assembler messages:
>> foo.c:2: Error: `%rax' not allowed with `orb'
>>
>> The "q" constraint only has meanting on -m32 otherwise is treated as
>> "r".
>>
>> This is easily reproducible via
>Clang+CONFIG_STAGING=y+CONFIG_VT6656=m,
>> or Clang+allyesconfig.
>>
>> Keep the masking operation to appease sparse (`make C=1`), add back
>the
>> cast in order to properly select the proper 8b register alias.
>>
>>  [Nick: reworded]
>>
>> Cc: stable@...r.kernel.org
>> Cc: Jesse Brandeburg <jesse.brandeburg@...el.com>
>> Link: https://github.com/ClangBuiltLinux/linux/issues/961
>> Link:
>https://lore.kernel.org/lkml/20200504193524.GA221287@google.com/
>> Fixes: 1651e700664b4 ("x86: Fix bitops.h warning with a moved cast")
>> Reported-by: Sedat Dilek <sedat.dilek@...il.com>
>> Reported-by: kernelci.org bot <bot@...nelci.org>
>> Suggested-by: Andy Shevchenko <andriy.shevchenko@...el.com>
>> Suggested-by: Ilie Halip <ilie.halip@...il.com>
>> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
>> Signed-off-by: Sedat Dilek <sedat.dilek@...il.com>
>> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>> ---
>>  arch/x86/include/asm/bitops.h | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/bitops.h
>b/arch/x86/include/asm/bitops.h
>> index b392571c1f1d..139122e5b25b 100644
>> --- a/arch/x86/include/asm/bitops.h
>> +++ b/arch/x86/include/asm/bitops.h
>> @@ -54,7 +54,7 @@ arch_set_bit(long nr, volatile unsigned long *addr)
>>         if (__builtin_constant_p(nr)) {
>>                 asm volatile(LOCK_PREFIX "orb %1,%0"
>>                         : CONST_MASK_ADDR(nr, addr)
>> -                       : "iq" (CONST_MASK(nr) & 0xff)
>> +                       : "iq" ((u8)(CONST_MASK(nr) & 0xff))
>
>I think a better fix would be to make CONST_MASK() return a u8 value
>rather than have to cast on every use.
>
>Also I question the need for the "q" constraint.  It was added in
>commit 437a0a54 as a workaround for GCC 3.4.4.  Now that the minimum
>GCC version is 4.6, is this still necessary?
>
>--
>Brian Gerst

Yes, "q" is needed on i386.
-- 
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