linux-kernel - Re: [PATCH 1/5] Kconfig: Add option for asm goto w/ tied outputs to workaround clang-13 bug

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Message-ID: <CAKwvOd=9nwR7z7wn50SU=mf5AywFLd95ZMH-EbYdHfbeHVvq1A@mail.gmail.com>
Date:   Tue, 1 Feb 2022 12:16:52 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        syzbot+6cde2282daa792c49ab8@...kaller.appspotmail.com
Subject: Re: [PATCH 1/5] Kconfig: Add option for asm goto w/ tied outputs to
 workaround clang-13 bug

On Mon, Jan 31, 2022 at 5:08 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Add a config option to guard (future) usage of asm_volatile_goto() that
> includes "tied outputs", i.e. "+" constraints that specify both an input
> and output parameter.  clang-13 has a bug[1] that causes compilation of
> such inline asm to fail, and KVM wants to use a "+m" constraint to
> implement a uaccess form of CMPXCHG[2].  E.g. the test code fails with
>
>   <stdin>:1:29: error: invalid operand in inline asm: '.long (${1:l}) - .'
>   int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }
>                             ^
>   <stdin>:1:29: error: unknown token in expression
>   <inline asm>:1:9: note: instantiated into assembly here
>           .long () - .
>                  ^
>   2 errors generated.
>
> on clang-13, but passes on gcc (with appropriate asm goto support).  The
> bug is fixed in clang-14, but won't be backported to clang-13 as the
> changes are too invasive/risky.

LGTM.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

If you're going to respin the series, consider adding a comment in the
source along the lines of:
```
clang-14 and gcc-11 fixed this.
```
or w/e. This helps us find (via grep) and remove cruft when the
minimum supported compiler versions are updated.

Note: gcc-10 had a bug with the symbolic references to labels when
using tied constraints.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98096

Both compilers had bugs here, and it may be worth mentioning that in
the commit message.

>
> [1] https://github.com/ClangBuiltLinux/linux/issues/1512
> [2] https://lore.kernel.org/all/YfMruK8%2F1izZ2VHS@google.com
>
> Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  init/Kconfig | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index e9119bf54b1f..a206b21703be 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -77,6 +77,10 @@ config CC_HAS_ASM_GOTO_OUTPUT
>         depends on CC_HAS_ASM_GOTO
>         def_bool $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null)
>
> +config CC_HAS_ASM_GOTO_TIED_OUTPUT
> +       depends on CC_HAS_ASM_GOTO_OUTPUT
> +       def_bool $(success,echo 'int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }' | $CC -x c - -c -o /dev/null)
> +
>  config TOOLS_SUPPORT_RELR
>         def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(NM)" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh)
>
> --
> 2.35.0.rc2.247.g8bbb082509-goog
>


-- 
Thanks,
~Nick Desaulniers