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Date:   Tue, 05 May 2020 11:07:24 -0700
From:   hpa@...or.com
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
CC:     Sedat Dilek <sedat.dilek@...il.com>, 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>, 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@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86: bitops: fix build regression

On May 5, 2020 10:44:22 AM PDT, 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))
> 			: "memory");
> 	} else {
> 		asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0"
>@@ -74,7 +74,7 @@ arch_clear_bit(long nr, volatile unsigned long *addr)
> 	if (__builtin_constant_p(nr)) {
> 		asm volatile(LOCK_PREFIX "andb %1,%0"
> 			: CONST_MASK_ADDR(nr, addr)
>-			: "iq" (CONST_MASK(nr) ^ 0xff));
>+			: "iq" ((u8)(CONST_MASK(nr) ^ 0xff)));
> 	} else {
> 		asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
> 			: : RLONG_ADDR(addr), "Ir" (nr) : "memory");

Drop & 0xff and change ^ 0xff to ~.

The redundancy is confusing.
-- 
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