Re: [PATCH 0/4] Section alignment issues?

From: Luis Chamberlain <mcgrof@kernel.org>
To: deller@kernel.org
Cc: linux-kernel@vger.kernel.org,
	Masahiro Yamada <masahiroy@kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-modules@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH 0/4] Section alignment issues?
Date: Wed, 20 Dec 2023 11:40:28 -0800	[thread overview]
Message-ID: <ZYNDLEzkjfrpgu7U@bombadil.infradead.org> (raw)
In-Reply-To: <ZYIKmQj0H1YAJWlz@bombadil.infradead.org>

On Tue, Dec 19, 2023 at 01:26:49PM -0800, Luis Chamberlain wrote:
> On Wed, Nov 22, 2023 at 11:18:10PM +0100, deller@kernel.org wrote:
> > From: Helge Deller <deller@gmx.de>
> > My questions:
> > - Am I wrong with my analysis?
> 
> This would typically of course depend on the arch, but whether it helps
> is what I would like to see with real numbers rather then speculation.
> Howeer, I don't expect some of these are hot paths except maybe the
> table lookups. So could you look at some perf stat differences
> without / with alignment ? Other than bootup live patching would be
> a good test case. We have selftest for modules, the script in selftests
> tools/testing/selftests/kmod/kmod.sh is pretty aggressive, but the live
> patching tests might be better suited.
> 
> > - What does people see on other architectures?
> > - Does it make sense to add a compile- and runtime-check, like the patch below, to the kernel?
> 
> The chatty aspects really depend on the above results.
> 
> Aren't there some archs where an unaligned access would actually crash?
> Why hasn't that happened?

I've gone down through memory lane and we have discussed this before.

It would seem this misalignment should not affect performance, and this
should not be an issue unless you have a buggy exception hanlder. We
actually ran into one before. Please refer to merge commit

e74acdf55da6649dd30be5b621a93b71cbe7f3f9

  Luis