linux-kernel - Re: [RFC v3 1/5] documentation: Discourage alignment assumptions

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Message-Id: <b80e06b8-e568-408b-8132-99565c50a0ff@app.fastmail.com>
Date: Thu, 16 Oct 2025 15:30:19 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Finn Thain" <fthain@...ux-m68k.org>,
 "Peter Zijlstra" <peterz@...radead.org>, "Will Deacon" <will@...nel.org>
Cc: "Andrew Morton" <akpm@...ux-foundation.org>,
 "Boqun Feng" <boqun.feng@...il.com>, "Jonathan Corbet" <corbet@....net>,
 "Mark Rutland" <mark.rutland@....com>, linux-kernel@...r.kernel.org,
 Linux-Arch <linux-arch@...r.kernel.org>,
 "Geert Uytterhoeven" <geert@...ux-m68k.org>, linux-m68k@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [RFC v3 1/5] documentation: Discourage alignment assumptions

 On Wed, Oct 8, 2025, at 00:19, Finn Thain wrote:
> Discourage assumptions that simply don't hold for all Linux ABIs.
> Exceptions to the natural alignment rule for scalar types include
> long long on i386 and sh.
> ---

I think both of the paragraphs you remove are still correct and I
would not remove them:

>  Documentation/core-api/unaligned-memory-access.rst | 7 -------
>  1 file changed, 7 deletions(-)
>
> diff --git a/Documentation/core-api/unaligned-memory-access.rst 
> b/Documentation/core-api/unaligned-memory-access.rst
> index 5ceeb80eb539..1390ce2b7291 100644
> --- a/Documentation/core-api/unaligned-memory-access.rst
> +++ b/Documentation/core-api/unaligned-memory-access.rst
> 
> -When writing code, assume the target architecture has natural alignment
> -requirements.
> -

It is clearly important to not intentionally misalign variables
because that breaks on hardware that requires aligned data.

Assuming natural alignment is the safe choice here, but you
could change 'architecture' to 'hardware' here if you
think that is otherwise ambiguous.

> -Similarly, you can also rely on the compiler to align variables and function
> -parameters to a naturally aligned scheme, based on the size of the type of
> -the variable.

This also seems to refer to something else: even on m68k
and i386, scalar stack and .data variables have natural
alignment even though the ABI does not require that.

It's probably a good idea to list the specific exceptions to
the struct layout rules in the previous paragraph, e.g.

[
 Fortunately, the compiler understands the alignment constraints, so in the
 above case it would insert 2 bytes of padding in between field1 and field2.
 Therefore, for standard structure types you can always rely on the compiler
-to pad structures so that accesses to fields are suitably aligned (assuming
-you do not cast the field to a type of different length).
+to pad structures so that accesses to fields are suitably aligned for
+the CPU hardware.
+On all 64-bit architectures, this means that all scalar struct members
+are naturally aligned. However, some 32-bit ABIs including i386
+only align 64-bit members on 32-bit offsets, and m68k uses at most
+16-bit alignment.
]

      Arnd