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

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Message-ID: <c4922789-0561-46bd-814b-1cd0253d3562@gmx.de>
Date: Fri, 22 Dec 2023 10:13:44 +0100
From: Helge Deller <deller@....de>
To: Luis Chamberlain <mcgrof@...nel.org>, deller@...nel.org
Cc: linux-kernel@...r.kernel.org, Masahiro Yamada <masahiroy@...nel.org>,
 Arnd Bergmann <arnd@...db.de>, linux-modules@...r.kernel.org,
 linux-arch@...r.kernel.org
Subject: Re: [PATCH 0/4] Section alignment issues?

On 12/20/23 20:40, Luis Chamberlain wrote:
> On Tue, Dec 19, 2023 at 01:26:49PM -0800, Luis Chamberlain wrote:
>> On Wed, Nov 22, 2023 at 11:18:10PM +0100, deller@...nel.org wrote:
>>> From: Helge Deller <deller@....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

Yes, this is the second time I stumbled over this issue.
But let's continue discussing in the other mail thread...

Helge