linux-kernel - Re: [PATCH RFC bpf-next v1 2/4] bpf: Introduce load-acquire and store-release instructions

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Message-ID: <Z25xTKrgR0j6U3qA@google.com>
Date: Fri, 27 Dec 2024 09:20:12 +0000
From: Peilin Ye <yepeilin@...gle.com>
To: Xu Kuohai <xukuohai@...weicloud.com>
Cc: bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...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>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Puranjay Mohan <puranjay@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Quentin Monnet <qmo@...nel.org>,
	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
	Josh Don <joshdon@...gle.com>, Barret Rhoden <brho@...gle.com>,
	Neel Natu <neelnatu@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>,
	David Vernet <dvernet@...a.com>,
	Dave Marchevsky <davemarchevsky@...a.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next v1 2/4] bpf: Introduce load-acquire and
 store-release instructions

On Fri, Dec 27, 2024 at 12:23:22AM +0000, Peilin Ye wrote:
>         if (off) {
> -               emit_a64_mov_i(true, tmp, off, ctx);
> -               emit(A64_ADD(true, tmp, tmp, ptr), ctx);
> -               ptr = tmp;
> +               if (is_addsub_imm(off)) {
> +                       emit(A64_ADD_I(true, ptr, ptr, off), ctx);
                                               ~~~

> +               } else if (is_addsub_imm(-off)) {
> +                       emit(A64_SUB_I(true, ptr, ptr, -off), ctx);
                                               ~~~

No, I must not write to the 'ptr' register here.

> +               } else {
> +                       emit_a64_mov_i(true, tmp, off, ctx);
> +                       emit(A64_ADD(true, tmp, tmp, ptr), ctx);
> +                       ptr = tmp;
> +               }
>         }

I will do this instead:

--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -658,8 +658,14 @@ static int emit_atomic_load_store(const struct bpf_insn *insn, struct jit_ctx *c
                ptr = dst;

        if (off) {
-               emit_a64_mov_i(true, tmp, off, ctx);
-               emit(A64_ADD(true, tmp, tmp, ptr), ctx);
+               if (is_addsub_imm(off)) {
+                       emit(A64_ADD_I(true, tmp, ptr, off), ctx);
+               } else if (is_addsub_imm(-off)) {
+                       emit(A64_SUB_I(true, tmp, ptr, -off), ctx);
+               } else {
+                       emit_a64_mov_i(true, tmp, off, ctx);
+                       emit(A64_ADD(true, tmp, tmp, ptr), ctx);
+               }
                ptr = tmp;
        }
        if (arena) {

Thanks,
Peilin Ye