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

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Message-ID: <20251014112359.451d8058@pumpkin>
Date: Tue, 14 Oct 2025 11:23:59 +0100
From: David Laight <david.laight.linux@...il.com>
To: Finn Thain <fthain@...ux-m68k.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Will Deacon <will@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Boqun Feng
 <boqun.feng@...il.com>, Jonathan Corbet <corbet@....net>, Mark Rutland
 <mark.rutland@....com>, Arnd Bergmann <arnd@...db.de>,
 linux-kernel@...r.kernel.org, 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, 08 Oct 2025 09:19:20 +1100
Finn Thain <fthain@...ux-m68k.org> 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.
> ---
>  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
> @@ -40,9 +40,6 @@ The rule mentioned above forms what we refer to as natural alignment:
>  When accessing N bytes of memory, the base memory address must be evenly
>  divisible by N, i.e. addr % N == 0.
>  
> -When writing code, assume the target architecture has natural alignment
> -requirements.

I think I'd be more explicit, perhaps:
Note that not all architectures align 64bit items on 8 byte boundaries or
even 32bit items on 4 byte boundaries.

	David

> -
>  In reality, only a few architectures require natural alignment on all sizes
>  of memory access. However, we must consider ALL supported architectures;
>  writing code that satisfies natural alignment requirements is the easiest way
> @@ -103,10 +100,6 @@ 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).
>  
> -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.
> -
>  At this point, it should be clear that accessing a single byte (u8 or char)
>  will never cause an unaligned access, because all memory addresses are evenly
>  divisible by one.