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From: Arnd Bergmann <arnd@kernel.org>
To: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
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: Fri, 14 May 2021 14:22:58 +0200	[thread overview]
Message-ID: <CAK8P3a1oO_moABCtNqLkM9ccVh9c=andfz+qiSucTCXcqJkYVA@mail.gmail.com> (raw)
In-Reply-To: <3d70eb2a-2969-197e-63e8-f3e0a6a8ddd8@physik.fu-berlin.de>

On Fri, May 14, 2021 at 12:34 PM John Paul Adrian Glaubitz
<glaubitz@physik.fu-berlin.de> wrote:
>
> Hi Arnd!
>
> On 5/14/21 12:00 PM, Arnd Bergmann wrote:
> > Unlike every other architecture, sh4a uses an inline asm implementation
> > for get_unaligned(). I have shown that this produces better object
> > code than the asm-generic version. However, there are very few users of
> > arch/sh/ overall, and most of those seem to use sh4 rather than sh4a CPU
> > cores, so it seems not worth keeping the complexity in the architecture
> > independent code.
>
> 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.

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.

Not sure if there are gcc developers that have an active interest in sh4a
support any more.

      Arnd

  reply	other threads:[~2021-05-14 12:24 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 [this message]
2021-05-15 15:36       ` John Paul Adrian Glaubitz
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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