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

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Message-ID: <Z6qC303CzfUMN8nV@google.com>
Date: Mon, 10 Feb 2025 22:51:11 +0000
From: Peilin Ye <yepeilin@...gle.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.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

Hi Alexei,

On Sat, Feb 08, 2025 at 07:46:54PM -0800, Alexei Starovoitov wrote:
> > Got it!  In v3, I'll change it back to:
> >
> >   #define BPF_LOAD_ACQ   0x10
> >   #define BPF_STORE_REL  0x20
> 
> why not 1 and 2 ?

I just realized that we can't do 1 and 2 because BPF_ADD | BPF_FETCH
also equals 1.

> All other bits are reserved and the verifier will make sure they're zero

IOW, we can't tell if imm<4-7> is reserved or BPF_ADD (0x00).  What
would you suggest?  Maybe:

  #define BPF_ATOMIC_LD_ST 0x10

  #define BPF_LOAD_ACQ      0x1
  #define BPF_STORE_REL     0x2

?

> , so when/if we need to extend it then it wouldn't matter whether
> lower 4 bits are reserved or other bits.
> Say, we decide to support cmpwait_relaxed as a new insn.
> It can take the value 3 and arm64 JIT will map it to ldxr+wfe+...
> 
> Then with this new load_acq and cmpwait_relaxed we can efficiently
> implement both smp_cond_load_relaxed and smp_cond_load_acquire.

Thanks,
Peilin Ye