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

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Message-ID: <CAADnVQK1GqQKxdoM9e1Z92QK68GEjqgMnC36ooVgS1uUNiP6eg@mail.gmail.com>
Date: Thu, 9 Oct 2025 08:17:55 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: 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>, Arnd Bergmann <arnd@...db.de>, 
	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 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.