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

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Date:   Thu, 7 May 2020 09:32:24 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        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 Thu, May 7, 2020 at 3:02 AM <hpa@...or.com> wrote:
>
> 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.

I think the bug this worked around was that the compiler didn't detect
that CONST_MASK(nr) was also constant and doesn't need to be put into
a register.  The question is does that bug still exist on compiler
versions we care about?

--
Brian Gerst