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From: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
To: Arnd Bergmann <arnd@kernel.org>
Cc: linux-arch <linux-arch@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Vineet Gupta <vgupta@synopsys.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Rich Felker <dalias@libc.org>,
	Linux-sh list <linux-sh@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 03/13] sh: remove unaligned access for sh4a
Date: Sat, 15 May 2021 17:36:53 +0200	[thread overview]
Message-ID: <71b5d15d-7bd2-aa08-cc0a-3caccf9c66c8@physik.fu-berlin.de> (raw)
In-Reply-To: <CAK8P3a1oO_moABCtNqLkM9ccVh9c=andfz+qiSucTCXcqJkYVA@mail.gmail.com>

Hi Arnd!

On 5/14/21 2:22 PM, Arnd Bergmann wrote:
>> My Renesas SH4-Boards actually run an sh4a-Kernel, not an sh4-Kernel:
>>
>> root@tirpitz:~> uname -a
>> Linux tirpitz 5.11.0-rc4-00012-g10c03c5bf422 #161 PREEMPT Mon Jan 18 21:10:17 CET 2021 sh4a GNU/Linux
>> root@tirpitz:~>
>>
>> So, if this change reduces performance on sh4a, I would rather not merge it.
> 
> It only makes a difference in very specific scenarios in which unaligned
> accesses are done in a fast path, e.g. when forwarding network packet
> at a high rate on a big-endian kernel (little-endian kernels wouldn't run into
> this on IP headers). If you have a use case for this machine on which the
> you can show a performance regression, I can add a patch on top to put
> the optimized sh4a get_unaligned_le32() back. Dropping this patch
> altogether would make the series much more complex because most of
> the associated code gets removed in the end.

Hmm, okay. But why does code which sits below arch/sh have to be removed anyway?

I don't fully understand why it poses any maintenance burden/

> As I mentioned, supporting "movua" in the compiler likely has a much
> larger impact on performance, as it would also help in user space, and
> it should improve the networking case on little-endian kernels by replacing
> the four separate byte loads/shift pairs with a movua plus a byteswap.

The problem is that - at least in Debian - we use the sh4 baseline while the kernel
supports both sh4 and sh4a, so we can't use any of these instructions in userland at
the moment.

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer - glaubitz@debian.org
`. `'   Freie Universitaet Berlin - glaubitz@physik.fu-berlin.de
  `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913

  reply	other threads:[~2021-05-15 15:36 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-14 10:00 [PATCH v2 00/13] Unify asm/unaligned.h around struct helper Arnd Bergmann
2021-05-14 10:00 ` [PATCH v2 03/13] sh: remove unaligned access for sh4a Arnd Bergmann
2021-05-14 10:34   ` John Paul Adrian Glaubitz
2021-05-14 12:22     ` Arnd Bergmann
2021-05-15 15:36       ` John Paul Adrian Glaubitz [this message]
2021-05-15 20:10         ` Arnd Bergmann
2021-05-14 17:32 ` [PATCH v2 00/13] Unify asm/unaligned.h around struct helper Linus Torvalds
2021-05-14 18:51   ` Vineet Gupta
2021-05-14 19:22     ` Linus Torvalds
2021-05-14 19:45       ` Vineet Gupta
2021-05-14 20:19         ` Linus Torvalds
2021-05-14 19:31   ` Arnd Bergmann
2021-12-16 17:29 ` Ard Biesheuvel
2021-12-16 17:42   ` Linus Torvalds
2021-12-16 17:49   ` David Laight
2021-12-16 18:56   ` Segher Boessenkool
2021-12-17 12:34     ` David Laight
2021-12-17 13:35       ` Segher Boessenkool

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