netdev - Re: [PATCH bpf-next 4/4] riscv, bpf: Mixing bpf2bpf and tailcalls

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Date: Tue, 19 Sep 2023 19:23:07 +0800
From: Pu Lehui <pulehui@...wei.com>
To: Conor Dooley <conor@...nel.org>, Pu Lehui <pulehui@...weicloud.com>
CC: <bpf@...r.kernel.org>, <linux-riscv@...ts.infradead.org>,
	<netdev@...r.kernel.org>, Björn Töpel
	<bjorn@...nel.org>, 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
	<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>
Subject: Re: [PATCH bpf-next 4/4] riscv, bpf: Mixing bpf2bpf and tailcalls



On 2023/9/19 18:04, Conor Dooley wrote:
> On Tue, Sep 19, 2023 at 11:57:11AM +0800, Pu Lehui wrote:
>> From: Pu Lehui <pulehui@...wei.com>
>>
>> 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.
>>
>> Tests test_bpf.ko and test_verifier have passed, as well as the relative
>> testcases of test_progs*.
>>
>> Signed-off-by: Pu Lehui <pulehui@...wei.com>
> 
> Breaks the build:
> ../arch/riscv/net/bpf_jit_comp64.c:846:14: error: use of undeclared identifier 'BPF_TRAMP_F_TAIL_CALL_CTX'
> 

Hi Conor,

BPF_TRAMP_F_TAIL_CALL_CTX rely on commit [0], and it has been merged 
into bpf-next tree.

Link: 
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=2b5dcb31a19a2e0acd869b12c9db9b2d696ef544 
[0]

> Thanks,
> Conor.