From: Herbert Xu <herbert@gondor.apana.org.au>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
Xiaodong Liu <xiaodong.liu@intel.com>,
Megha Dey <megha.dey@intel.com>,
linux-crypto@vger.kernel.org, x86@kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/crypto: make constants readonly, allow linker to merge them
Date: Mon, 23 Jan 2017 23:00:51 +0800 [thread overview]
Message-ID: <20170123150051.GG20806@gondor.apana.org.au> (raw)
In-Reply-To: <20170119213304.18140-1-dvlasenk@redhat.com>
On Thu, Jan 19, 2017 at 10:33:04PM +0100, Denys Vlasenko wrote:
> A lot of asm-optimized routines in arch/x86/crypto/ keep its
> constants in .data. This is wrong, they should be on .rodata.
>
> Mnay of these constants are the same in different modules.
> For example, 128-bit shuffle mask 0x000102030405060708090A0B0C0D0E0F
> exists in at least half a dozen places.
>
> There is a way to let linker merge them and use just one copy.
> The rules are as follows: mergeable objects of different sizes
> should not share sections. You can't put them all in one .rodata
> section, they will lose "mergeability".
>
> GCC puts its mergeable constants in ".rodata.cstSIZE" sections,
> or ".rodata.cstSIZE.<object_name>" if -fdata-sections is used.
> This patch does the same:
>
> .section .rodata.cst16.SHUF_MASK, "aM", @progbits, 16
>
> It is important that all data in such section consists of
> 16-byte elements, not larger ones, and there are no implicit
> use of one element from another.
>
> When this is not the case, use non-mergeable section:
>
> .section .rodata[.VAR_NAME], "a", @progbits
>
> This reduces .data by ~15 kbytes:
>
> text data bss dec hex filename
> 11097415 2705840 2630712 16433967 fac32f vmlinux-prev.o
> 11112095 2690672 2630712 16433479 fac147 vmlinux.o
>
> Merged objects are visible in System.map:
>
> ffffffff81a28810 r POLY
> ffffffff81a28810 r POLY
> ffffffff81a28820 r TWOONE
> ffffffff81a28820 r TWOONE
> ffffffff81a28830 r PSHUFFLE_BYTE_FLIP_MASK <- merged regardless of
> ffffffff81a28830 r SHUF_MASK <------------- the name difference
> ffffffff81a28830 r SHUF_MASK
> ffffffff81a28830 r SHUF_MASK
> ..
> ffffffff81a28d00 r K512 <- merged three identical 640-byte tables
> ffffffff81a28d00 r K512
> ffffffff81a28d00 r K512
>
> Use of object names in section name suffixes is not strictly necessary,
> but might help if someday link stage will use garbage collection
> to eliminate unused sections (ld --gc-sections).
>
> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Patch applied. Thanks.
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
prev parent reply other threads:[~2017-01-23 15:01 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-19 21:33 [PATCH] x86/crypto: make constants readonly, allow linker to merge them Denys Vlasenko
2017-01-19 23:09 ` Thomas Gleixner
2017-01-23 15:21 ` Denys Vlasenko
2017-01-23 15:00 ` Herbert Xu [this message]
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