From: George Guo <dongtai.guo@linux.dev>
To: yangtiezhu@loongson.cn
Cc: chenhuacai@kernel.org, hengqi.chen@gmail.com, ast@kernel.org,
daniel@iogearbox.net, andrii@kernel.org, kernel@xen0n.name,
martin.lau@linux.dev, eddyz87@gmail.com, memxor@gmail.com,
song@kernel.org, yonghong.song@linux.dev, jolsa@kernel.org,
guodongtai@kylinos.cn, bpf@vger.kernel.org,
loongarch@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf-next v2 02/11] LoongArch: BPF: Support internal-only MOV to resolve per-CPU addrs
Date: Tue, 7 Jul 2026 18:22:11 +0800 [thread overview]
Message-ID: <20260707102211.342727-1-dongtai.guo@linux.dev> (raw)
In-Reply-To: <bcdfd520-8f88-b8d9-37e1-52e0368d40d3@loongson.cn>
> How to test? Did you test under !CONFIG_SMP?
Good question -- here's why skipping the $r21 add under !CONFIG_SMP is
correct, not just untested:
1. $r21 is unconditionally zeroed at the earliest point in boot,
regardless of CONFIG_SMP:
arch/loongarch/kernel/head.S:75
/* GPR21 used for percpu base (runtime), initialized as 0 */
move u0, zero
2. The only other writer, set_my_cpu_offset(), lives in
arch/loongarch/kernel/smp.c, which is `obj-$(CONFIG_SMP) += smp.o` --
i.e. not even compiled into a !CONFIG_SMP kernel. So on !CONFIG_SMP,
$r21 stays exactly 0 for the kernel's entire lifetime; nothing ever
touches it again after head.S.
This also isn't a LoongArch-specific judgment call -- it's exactly the
pattern x86's own upstream JIT uses for the identical instruction:
arch/x86/net/bpf_jit_comp.c, insn_is_mov_percpu_addr():
EMIT_mov(dst_reg, src_reg);
#ifdef CONFIG_SMP
/* add <dst>, gs:[<off>] */
...
#endif
Thanks,
George
next prev parent reply other threads:[~2026-07-07 10:22 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-07-02 2:23 [PATCH bpf-next v2 00/11] LoongArch: BPF: arena features, exceptions, private stack and may_goto George Guo
2026-07-02 2:23 ` [PATCH bpf-next v2 01/11] LoongArch: BPF: Fix tail call count pointer offset for arena programs George Guo
2026-07-02 2:35 ` sashiko-bot
2026-07-06 4:55 ` Tiezhu Yang
2026-07-07 9:43 ` George Guo
2026-07-07 10:12 ` Tiezhu Yang
2026-07-02 2:23 ` [PATCH bpf-next v2 02/11] LoongArch: BPF: Support internal-only MOV to resolve per-CPU addrs George Guo
2026-07-06 6:46 ` Tiezhu Yang
2026-07-07 10:22 ` George Guo [this message]
2026-07-02 2:23 ` [PATCH bpf-next v2 03/11] LoongArch: BPF: Add timed may_goto support George Guo
2026-07-02 2:23 ` [PATCH bpf-next v2 04/11] LoongArch: BPF: Add private stack support George Guo
2026-07-02 2:23 ` [PATCH bpf-next v2 05/11] LoongArch: BPF: Add exceptions (bpf_throw) support George Guo
2026-07-02 2:39 ` sashiko-bot
2026-07-02 2:23 ` [PATCH bpf-next v2 06/11] LoongArch: BPF: Support sign-extending loads from arena George Guo
2026-07-02 2:23 ` [PATCH bpf-next v2 07/11] LoongArch: BPF: Support atomics on arena pointers George Guo
2026-07-02 2:48 ` sashiko-bot
2026-07-06 7:06 ` Tiezhu Yang
2026-07-07 10:55 ` George Guo
2026-07-02 2:23 ` [PATCH bpf-next v2 08/11] selftests/bpf: Enable struct_ops private stack test for LoongArch George Guo
2026-07-02 2:23 ` [PATCH bpf-next v2 09/11] selftests/bpf: Enable arena LDSX tests on LoongArch George Guo
2026-07-02 2:23 ` [PATCH bpf-next v2 10/11] selftests/bpf: Enable arena atomics " George Guo
2026-07-02 2:49 ` sashiko-bot
2026-07-02 2:23 ` [PATCH bpf-next v2 11/11] selftests/bpf: Add LoongArch deny list George Guo
2026-07-06 6:37 ` Tiezhu Yang
2026-07-03 10:11 ` [PATCH bpf-next v2 00/11] LoongArch: BPF: arena features, exceptions, private stack and may_goto Huacai Chen
2026-07-06 7:15 ` Tiezhu Yang
2026-07-07 11:25 ` George Guo
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