linux-kernel - Re: [PATCH v5 14/16] modules: Support extended MODVERSIONS info

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Message-ID: <ZxQabNukxBo3utbh@bombadil.infradead.org>
Date: Sat, 19 Oct 2024 13:45:32 -0700
From: Luis Chamberlain <mcgrof@...nel.org>
To: Helge Deller <deller@....de>
Cc: Matthew Maurer <mmaurer@...gle.com>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Petr Pavlu <petr.pavlu@...e.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Daniel Gomez <da.gomez@...sung.com>, masahiroy@...nel.org,
	ndesaulniers@...gle.com, ojeda@...nel.org, gary@...yguo.net,
	Michael Ellerman <mpe@...erman.id.au>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Benjamin Gray <bgray@...ux.ibm.com>,
	Naveen N Rao <naveen@...nel.org>, rust-for-linux@...r.kernel.org,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	neal@...pa.dev, marcan@...can.st, j@...nau.net,
	asahi@...ts.linux.dev, linux-modules@...r.kernel.org,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5 14/16] modules: Support extended MODVERSIONS info

On Thu, Oct 17, 2024 at 02:08:19PM +0200, Helge Deller wrote:
> Hi Luis,
> 
> On 10/17/24 01:21, Luis Chamberlain wrote:
> > That sounds great. Yeah, the above would be great to test. A while ago
> > I wrote a new modules selftests in order to test possible improvements
> > on find_symbol() but I also did this due to push the limits of the
> > numbers of symbols we could support. I wrote all this to also test the
> > possible 64-bit alignment benefits of __ksymtab_ sections on
> > architectures without CONFIG_HAVE_ARCH_PREL32_RELOCATIONS (e.g. ppc64,
> > ppc64le, parisc, s390x,...). [....]
> > 
> > I forget what we concluded on Helge Deller's alignement patches, I think
> > there was an idea on how to address the alignment through other means.
> > 
> > [0] https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux.git/log/?h=20241016-modules-symtab
> 
> I stumbled upon the unaligned-memory-access.rst document [1].
> Please read it, as it is a really good document, and the section
> "Why unaligned access is bad" states:
> It should be obvious from the above that if your code causes unaligned
> memory accesses to happen, your code will not work correctly on certain
> platforms and will cause performance problems on others.
> 
> With this in mind, you really should apply both of my alignment
> patches which you currently carry in [0].
> 
> For parisc I partly solved the issue by fixing the arch-specific kernel unalignment
> handler, but every time module sections are stored unaligned, it triggers
> performance degregation on parisc (and other sensitive platforms).
> 
> I suggest you apply them unconditionally.
> 
> Helge
> 
> [1]  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/core-api/unaligned-memory-access.rst

You're right, I've just referred to that doc and pushed to the new
linux modules [2] modules-next branch. This is also great timing so
that the work that is ongoing for Rust will take this into
consideration as well. I'll just post the test I wrote as separate
thing but it surely can be used to help test some of this later.

[2] git://git.kernel.org/pub/scm/linux/kernel/git/modules/linux.git

  Luis