netdev - Re: [PATCH bpf-next v3 0/4] Mixing bpf2bpf and tailcalls for RV64

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Message-ID: <1e7181e4-c4c5-d307-2c5c-5bf15016aa8a@iogearbox.net>
Date: Thu, 1 Feb 2024 11:56:34 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Pu Lehui <pulehui@...weicloud.com>, bpf@...r.kernel.org,
 linux-riscv@...ts.infradead.org, netdev@...r.kernel.org
Cc: Björn Töpel <bjorn@...nel.org>,
 Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Palmer Dabbelt <palmer@...belt.com>,
 Luke Nelson <luke.r.nels@...il.com>, Pu Lehui <pulehui@...wei.com>
Subject: Re: [PATCH bpf-next v3 0/4] Mixing bpf2bpf and tailcalls for RV64

On 2/1/24 9:33 AM, Pu Lehui wrote:
> In the current RV64 JIT, if we just don't initialize the TCC in subprog,
> the TCC can be propagated from the parent process to the subprocess, but
> the TCC of the parent process cannot be restored when the subprocess
> exits. Since the RV64 TCC is initialized before saving the callee saved
> registers into the stack, we cannot use the callee saved register to
> pass the TCC, otherwise the original value of the callee saved register
> will be destroyed. So we implemented mixing bpf2bpf and tailcalls
> similar to x86_64, i.e. using a non-callee saved register to transfer
> the TCC between functions, and saving that register to the stack to
> protect the TCC value. At the same time, we also consider the scenario
> of mixing trampoline.
> 
> In addition, some code cleans are also attached to this patchset.
> 
> Tests test_bpf.ko and test_verifier have passed, as well as the relative
> testcases of test_progs*.
> 
> v3:
> - Remove duplicate RV_REG_TCC load in epiloguei. (Björn Töpel)

Iiuc, this still needs a respin as per the ongoing discussions. Also,
if you have worked on BPF selftests which exercise the corner case
around a6, please include them in the series as well for coverage.

Thanks,
Daniel