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

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Message-ID: <CAADnVQL9PRCVEJBJ++TCRBWqshQLn7dz0SiJ6KWDYPeWLSK22w@mail.gmail.com>
Date: Thu, 9 Oct 2025 09:12:17 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Peter Zijlstra <peterz@...radead.org>, 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 9:02 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> 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.

I see. So this is a lesser evil.

Acked-by: Alexei Starovoitov <ast@...nel.org>

for the patch then.