From: Nadav Amit <namit@vmware.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: "keescook@chromium.org" <keescook@chromium.org>,
the arch/x86 maintainers <x86@kernel.org>,
"tglx@linutronix.de" <tglx@linutronix.de>,
Josh Poimboeuf <jpoimboe@redhat.com>,
"mingo@redhat.com" <mingo@redhat.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"hpa@zytor.com" <hpa@zytor.com>
Subject: Re: [RFC 5/8] x86: refcount: prevent gcc distortions
Date: Wed, 16 May 2018 16:44:00 +0000 [thread overview]
Message-ID: <8B7DE95F-5E57-44F0-90E0-CCBA35F98A07@vmware.com> (raw)
In-Reply-To: <5AFBDA6302000078001C31AC@prv1-mh.provo.novell.com>
Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 15.05.18 at 16:11, <namit@vmware.com> wrote:
>> --- a/arch/x86/include/asm/refcount.h
>> +++ b/arch/x86/include/asm/refcount.h
>> @@ -14,34 +14,43 @@
>> * central refcount exception. The fixup address for the exception points
>> * back to the regular execution flow in .text.
>> */
>> -#define _REFCOUNT_EXCEPTION \
>> - ".pushsection .text..refcount\n" \
>> - "111:\tlea %[counter], %%" _ASM_CX "\n" \
>> - "112:\t" ASM_UD2 "\n" \
>> - ASM_UNREACHABLE \
>> - ".popsection\n" \
>> - "113:\n" \
>> +
>> +asm ("\n"
>> + ".macro __REFCOUNT_EXCEPTION counter:vararg\n\t"
>> + ".pushsection .text..refcount\n"
>> + "111:\tlea \\counter, %" _ASM_CX "\n"
>> + "112:\t" ASM_UD2 "\n\t"
>> + ASM_UNREACHABLE
>> + ".popsection\n\t"
>> + "113:\n"
>> _ASM_EXTABLE_REFCOUNT(112b, 113b)
>> + ".endm");
>
> A few comments on assembly code formatting - while gas at present is
> relatively lax in this regard, I wouldn't exclude that there might be a
> more strict mode in the future, and that such a mode might eventually
> become the default. Furthermore these formatting aspects affect
> readability of the assembly produced, should anyone ever find a need
> to look at it (perhaps because of some breakage) - I certainly do every
> once in a while.
>
> Labels should be placed without any indentation (but of course there
> may be more than one on a line, in which case subsequent ones may
> of course be arbitrarily indented). Instructions and directives, otoh,
> should be placed with at least a single tab or space of indentation
> (unless preceded by a label, in which case the extra white space still
> helps readability).
Writing these patches, I looked at the generated assembly, and there did not
appear to be a standard. IIRC, .pushsection directives were not always
inlined. I will fix it according to your comments.
> I'm also not sure about the purpose of the leading plain newline here.
> gcc annotates code resulting from inline assembly anyway iirc, so
> proper visual separation should already be available.
Right. It was only to get the macro directive not tabulated, but as you
said, it should be tabulated, so I will remove it.
>
> If I was the maintainer of this code, I would also object to the
> mis-alignment your file scope asm()-s have ("asm (" is 5 characters,
> which doesn't equal a tab's width).
I tried many formats (including the one you propose), and eventually went
with the one that made checkpatch not yell at me. I will revert to the one
you propose, which makes most sense, and ignore checkpatch warnings.
Thanks,
Nadav
next prev parent reply other threads:[~2018-05-16 16:44 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-15 14:11 [RFC 0/8] Improving compiler inlining decisions Nadav Amit
2018-05-15 14:11 ` [RFC 1/8] x86: objtool: use asm macro for better compiler decisions Nadav Amit
2018-05-15 21:37 ` Josh Triplett
2018-05-15 21:53 ` Nadav Amit
2018-05-15 21:55 ` Josh Triplett
2018-05-15 14:11 ` [RFC 2/8] x86: bug: prevent gcc distortions Nadav Amit
2018-05-15 14:11 ` [RFC 3/8] x86: alternative: macrofy locks for better inlining Nadav Amit
2018-05-15 14:11 ` [RFC 4/8] x86: prevent inline distortion by paravirt ops Nadav Amit
2018-05-15 14:11 ` [RFC 5/8] x86: refcount: prevent gcc distortions Nadav Amit
2018-05-16 13:59 ` Kees Cook
2018-05-16 16:37 ` Nadav Amit
2018-05-15 14:11 ` [RFC 6/8] x86: removing unneeded new-lines Nadav Amit
2018-05-15 14:11 ` [RFC 7/8] ilog2: preventing compiler distortion due to big condition Nadav Amit
2018-05-15 14:11 ` [RFC 8/8] bitops: prevent compiler inline decision distortion Nadav Amit
2018-05-15 14:11 ` [RFC 0/8] Improving compiler inlining decisions Nadav Amit
2018-05-15 14:11 ` [RFC 1/8] x86: objtool: use asm macro for better compiler decisions Nadav Amit
2018-05-15 14:11 ` [RFC 2/8] x86: bug: prevent gcc distortions Nadav Amit
2018-05-15 14:11 ` [RFC 3/8] x86: alternative: macrofy locks for better inlining Nadav Amit
2018-05-15 14:11 ` [RFC 4/8] x86: prevent inline distortion by paravirt ops Nadav Amit
2018-05-15 14:11 ` [RFC 5/8] x86: refcount: prevent gcc distortions Nadav Amit
2018-05-16 7:14 ` Jan Beulich
2018-05-16 16:44 ` Nadav Amit [this message]
2018-05-17 7:18 ` Jan Beulich
2018-05-15 14:11 ` [RFC 6/8] x86: removing unneeded new-lines Nadav Amit
2018-05-15 14:11 ` [RFC 7/8] ilog2: preventing compiler distortion due to big condition Nadav Amit
2018-05-15 14:11 ` [RFC 8/8] bitops: prevent compiler inline decision distortion Nadav Amit
2018-05-16 14:09 ` Kees Cook
2018-05-15 22:14 ` [RFC 0/8] Improving compiler inlining decisions Nadav Amit
2018-05-16 3:48 ` Josh Poimboeuf
2018-05-16 4:30 ` Nadav Amit
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