linux-kernel - Re: [PATCH bpf-next v2] riscv, bpf: Add kfunc support for RV64

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Date:   Tue, 21 Feb 2023 09:47:03 +0800
From:   Pu Lehui <pulehui@...weicloud.com>
To:     Björn Töpel <bjorn@...nel.org>,
        bpf@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Cc:     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>,
        Pu Lehui <pulehui@...wei.com>
Subject: Re: [PATCH bpf-next v2] riscv, bpf: Add kfunc support for RV64



On 2023/2/20 22:34, Björn Töpel wrote:
> Pu Lehui <pulehui@...weicloud.com> writes:
> 
>> From: Pu Lehui <pulehui@...wei.com>
>>
>> As another important missing piece of RV64 JIT, kfunc allow bpf programs
>> call kernel functions. For now, RV64 is sufficient to enable it.
> 
> Thanks Lehui!
> 
> Maybe we can reword/massage the commit message a bit? What do you think
> about something like:
> 
> "Now that the BPF trampoline is supported by RISC-V, it is possible to
> use BPF programs with kfunc calls.
> 

kfunc and bpf trampoline are functionally independent. kfunc [1], like 
bpf helper functions, allows bpf programs to call exported kernel 
functions, while bpf trampoline provides a more efficient way than 
kprobe to act as a mediator between kernel functions and bpf programs, 
and between bpf programs.

In fact, it was already supported before the bpf trampoline 
implementation, I just turned it on. As for RV32 kfunc, it needs to do 
some registers parsing.

[1] https://lore.kernel.org/bpf/20210325015124.1543397-1-kafai@fb.com/

> Note that the trampoline functionality is only supported by RV64.
> 
> Add bpf_jit_supports_kfunc_call() to the 64-bit JIT."
> 
> 
> Björn