From: mkl@pengutronix•de (Marc Kleine-Budde)
To: linux-arm-kernel@lists•infradead.org
Subject: DWord alignment on ARMv7
Date: Thu, 3 Mar 2016 23:27:11 +0100 [thread overview]
Message-ID: <56D8BA3F.7050508@pengutronix.de> (raw)
Hello, (now sending to linux-arm-kernel)
I'm using btrfs on am ARMv7 and it turns out, that the kernel has to
fixup a lot of kernel originated alignment issues.
See /proc/cpu/alignment (~4h of uptime):
> System: 22304815 (btrfs_get_token_64+0x13c/0x148 [btrfs])
For example, when compiling the kernel on a btrfs volume the counter
increases by 100...1000 per second.
The function shown "btrfs_get_token_64()" is defined here:
> http://lxr.free-electrons.com/source/fs/btrfs/struct-funcs.c#L53
...it already uses get_unaligned_leXX accessors.
Quoting a comment in arch/arm/mm/alignment.c:
* ARMv6 and later CPUs can perform unaligned accesses for
* most single load and store instructions up to word size.
* LDM, STM, LDRD and STRD still need to be handled.
But on a 32bit ARMv7 64bits are not word-sized.
Is the exception and fixup overhead neglectable? Do we have to introduce
something like HAVE_EFFICIENT_UNALIGNED_64BIT_ACCESS?
regards,
Marc
--
Pengutronix e.K. | Marc Kleine-Budde |
Industrial Linux Solutions | Phone: +49-231-2826-924 |
Vertretung West/Dortmund | Fax: +49-5121-206917-5555 |
Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de |
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next reply other threads:[~2016-03-03 22:27 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-03 22:27 Marc Kleine-Budde [this message]
2016-03-03 23:54 ` DWord alignment on ARMv7 Will Deacon
2016-03-04 8:01 ` btrfs_get_token_64() alignment problem on ARM (was: Re: DWord alignment on ARMv7) Marc Kleine-Budde
2016-03-04 9:16 ` David Sterba
2016-03-04 10:48 ` DWord alignment on ARMv7 Ard Biesheuvel
2016-03-04 11:02 ` Russell King - ARM Linux
2016-03-04 11:14 ` Ard Biesheuvel
2016-03-04 11:19 ` Marc Kleine-Budde
2016-03-04 11:26 ` Ard Biesheuvel
2016-03-04 11:38 ` Arnd Bergmann
2016-03-04 11:44 ` Ard Biesheuvel
2016-03-04 13:30 ` Arnd Bergmann
2016-03-04 13:33 ` Ard Biesheuvel
2016-03-04 13:46 ` Russell King - ARM Linux
2016-03-04 14:41 ` Arnd Bergmann
2016-03-04 14:56 ` Russell King - ARM Linux
2016-03-04 15:49 ` Arnd Bergmann
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