linux-kernel - Re: [PATCH bpf-next v2 4/9] bpf: Introduce load-acquire and store-release instructions

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Message-ID: <CAADnVQKZ=pjXjyzB8tJj5Gen4odcj5H5JhXyRtVgphTEDCisTg@mail.gmail.com>
Date: Fri, 14 Feb 2025 19:04:13 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Peilin Ye <yepeilin@...gle.com>
Cc: bpf <bpf@...r.kernel.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, bpf@...f.org, 
	Xu Kuohai <xukuohai@...weicloud.com>, Eduard Zingerman <eddyz87@...il.com>, 
	David Vernet <void@...ifault.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, 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>, Jonathan Corbet <corbet@....net>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Puranjay Mohan <puranjay@...nel.org>, 
	Ilya Leoshkevich <iii@...ux.ibm.com>, Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Quentin Monnet <qmo@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	Ihor Solodrai <ihor.solodrai@...ux.dev>, Yingchi Long <longyingchi24s@....ac.cn>, 
	Josh Don <joshdon@...gle.com>, Barret Rhoden <brho@...gle.com>, Neel Natu <neelnatu@...gle.com>, 
	Benjamin Segall <bsegall@...gle.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 4/9] bpf: Introduce load-acquire and
 store-release instructions

On Fri, Feb 14, 2025 at 6:34 PM Peilin Ye <yepeilin@...gle.com> wrote:
>
> > On Wed, Feb 12, 2025 at 09:55:43PM -0800, Alexei Starovoitov wrote:
> > > How about:
> > > #define BPF_LOAD_ACQ 2
> > > #define BPF_STORE_REL 3
> > >
> > > and only use them with BPF_MOV like
> > >
> > > imm = BPF_MOV | BPF_LOAD_ACQ - is actual load acquire
> > > imm = BPF_MOV | BPF_STORE_REL - release
>
> Based on everything discussed, should we proceed with the above
> suggestion?  Specifically:
>
>   #define BPF_LD_ST     BPF_MOV /* 0xb0 */

The aliasing still bothers me.
I hated doing it when going from cBPF to eBPF,
but we only had 8-bit to work with.
Here we have 32-bit.
Aliases make disassemblers trickier, since value no longer
translates to string as-is. It depends on the context.
There is probably no use for BPF_MOV operation in atomic
context, but by reusing BPF_ADD, BPF_XOR, etc in atomic
we signed up ourselves for all of alu ops.
That's why BPF_XCHG and BPF_CMPXCHG are outside
of alu op range.

So my preference is to do:
#define BPF_LOAD_ACQ 0x100
#define BPF_STORE_REL 0x110
#define BPF_CMPWAIT_RELAXED   0x120

and keep growing it.
We burn the first nibble, but so be it.