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

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Message-ID: <f5f939ae-f966-37ba-369d-be147c0642a3@linux-m68k.org>
Date: Wed, 15 Oct 2025 18:40:39 +1100 (AEDT)
From: Finn Thain <fthain@...ux-m68k.org>
To: David Laight <david.laight.linux@...il.com>
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 Tue, 14 Oct 2025, David Laight wrote:

> 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.
> 

That's what the next para is alluding to...

> > 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 
> > to achieve full portability.

How about this?

"In reality, only a few architectures require natural alignment for all 
sizes of memory access. That is, not all architectures need 64-bit values 
to be aligned on 8-byte boundaries and 32-bit values on 4-byte boundaries. 
However, when writing code intended to achieve full portability, we must 
consider all supported architectures."

> > @@ -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.
> 
>