linux-kernel - Re: [RFC v3 2/5] bpf: Explicitly align bpf_res_spin

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Message-Id: <b9ab4c28-52c8-4fa7-85cb-109ef4c0d7f4@app.fastmail.com>
Date: Thu, 09 Oct 2025 18:01:51 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Alexei Starovoitov" <alexei.starovoitov@...il.com>,
 "Peter Zijlstra" <peterz@...radead.org>
Cc: "Finn Thain" <fthain@...ux-m68k.org>, "Will Deacon" <will@...nel.org>,
 "Alexei Starovoitov" <ast@...nel.org>,
 "Daniel Borkmann" <daniel@...earbox.net>,
 "Andrii Nakryiko" <andrii@...nel.org>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Boqun Feng" <boqun.feng@...il.com>, "Jonathan Corbet" <corbet@....net>,
 "Mark Rutland" <mark.rutland@....com>, LKML <linux-kernel@...r.kernel.org>,
 Linux-Arch <linux-arch@...r.kernel.org>,
 "Geert Uytterhoeven" <geert@...ux-m68k.org>, linux-m68k@...r.kernel.org,
 "Martin KaFai Lau" <martin.lau@...ux.dev>,
 "Eduard Zingerman" <eddyz87@...il.com>, "Song Liu" <song@...nel.org>,
 "Yonghong Song" <yonghong.song@...ux.dev>,
 "John Fastabend" <john.fastabend@...il.com>, "KP Singh" <kpsingh@...nel.org>,
 "Stanislav Fomichev" <sdf@...ichev.me>, "Hao Luo" <haoluo@...gle.com>,
 "Jiri Olsa" <jolsa@...nel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [RFC v3 2/5] bpf: Explicitly align bpf_res_spin_lock

On Thu, Oct 9, 2025, at 17:17, Alexei Starovoitov wrote:
> On Thu, Oct 9, 2025 at 12:02 AM Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> On Wed, Oct 08, 2025 at 07:10:13PM -0700, Alexei Starovoitov wrote:
>>
>> > Are you saying 'int' on m68k is not 4 byte aligned by default,
>> > so you have to force 4 byte align?
>>
>> This; m68k has u16 alignment, just to keep life interesting I suppose
>> :-)
>
> It's not "interesting". It adds burden to the rest of the kernel
> for this architectural quirk.
> Linus put the foot down for big-endian on arm64 and riscv.
> We should do the same here.
> x86 uses -mcmodel=kernel for 64-bit and -mregparm=3 for 32-bit.
> m68k can do the same.
> They can adjust the compiler to make 'int' 4 byte aligned under some
> compiler flag. The kernel is built standalone, so it doesn't have
> to conform to native calling convention or anything else.

I agree that building the kernel with -malign-int makes a lot
of sense here, there is even a project to rebuild the entire
user space with the same flag.

However, changing either the kernel or userspace to build with
-malign-int also has its cost, since for ABI compatibility
reasons any include/uapi/*/*.h header that defines a structure
with a misaligned word needs a custom annotation in order to
still define the layout to be the same as before, and the
annotations do complicate the common headers.

See 
https://lore.kernel.org/all/534e8ff8-70cb-4b78-b0b4-f88645bd180a@app.fastmail.com/
for a list of structures that likely need to be annotated,
and the thread around it for more of the nasty details that
make this nontrivial.

       Arnd