From: Ingo Molnar <mingo@kernel.org>
To: Alexander Potapenko <glider@google.com>
Cc: paulmck@linux.ibm.com, hpa@zytor.com, peterz@infradead.org,
linux-kernel@vger.kernel.org, dvyukov@google.com,
jyknight@google.com, x86@kernel.org, mingo@redhat.com,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Linus Torvalds <torvalds@linux-foundation.org>,
Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH v2] x86/asm: fix assembly constraints in bitops
Date: Fri, 5 Apr 2019 11:39:31 +0200 [thread overview]
Message-ID: <20190405093931.GA28890@gmail.com> (raw)
In-Reply-To: <20190402112813.193378-1-glider@google.com>
* Alexander Potapenko <glider@google.com> wrote:
> 1. Use memory clobber in bitops that touch arbitrary memory
>
> Certain bit operations that read/write bits take a base pointer and an
> arbitrarily large offset to address the bit relative to that base.
> Inline assembly constraints aren't expressive enough to tell the
> compiler that the assembly directive is going to touch a specific memory
> location of unknown size, therefore we have to use the "memory" clobber
> to indicate that the assembly is going to access memory locations other
> than those listed in the inputs/outputs.
> To indicate that BTR/BTS instructions don't necessarily touch the first
> sizeof(long) bytes of the argument, we also move the address to assembly
> inputs.
>
> This particular change leads to size increase of 124 kernel functions in
> a defconfig build. For some of them the diff is in NOP operations, other
> end up re-reading values from memory and may potentially slow down the
> execution. But without these clobbers the compiler is free to cache
> the contents of the bitmaps and use them as if they weren't changed by
> the inline assembly.
>
> 2. Use byte-sized arguments for operations touching single bytes.
>
> Passing a long value to ANDB/ORB/XORB instructions makes the compiler
> treat sizeof(long) bytes as being clobbered, which isn't the case. This
> may theoretically lead to worse code in the case of heavy optimization.
>
> Signed-off-by: Alexander Potapenko <glider@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Paul E. McKenney <paulmck@linux.ibm.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: James Y Knight <jyknight@google.com>
> ---
> v2:
> -- renamed the patch
> -- addressed comment by Peter Zijlstra: don't use "+m" for functions
> returning void
> -- fixed input types for operations touching single bytes
> ---
> arch/x86/include/asm/bitops.h | 41 +++++++++++++++--------------------
> 1 file changed, 18 insertions(+), 23 deletions(-)
I'm wondering what the primary motivation for the patch is:
- Does it fix an actual miscompilation, or only a theoretical miscompilation?
- If it fixes an existing miscompilation:
- Does it fix a miscompilation triggered by current/future versions of GCC?
- Does it fix a miscompilation triggered by current/future versions of Clang?
- Also, is the miscompilation triggered by 'usual' kernel configs, or
does it require exotics such as weird debug options or GCC plugins,
etc?
I.e. a bit more context would be useful.
Thanks,
Ingo
next prev parent reply other threads:[~2019-04-05 9:39 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-02 11:28 [PATCH v2] x86/asm: fix assembly constraints in bitops Alexander Potapenko
2019-04-02 11:33 ` Alexander Potapenko
2019-04-02 11:45 ` David Laight
2019-04-02 12:35 ` Alexander Potapenko
2019-04-02 12:37 ` Alexander Potapenko
2019-04-05 9:39 ` Ingo Molnar [this message]
2019-04-05 11:12 ` David Laight
2019-04-05 11:53 ` Alexander Potapenko
2019-04-06 8:20 ` Ingo Molnar
2019-04-06 8:46 ` [tip:x86/urgent] x86/asm: Use stricter " tip-bot for Alexander Potapenko
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