Message-ID: <80e69e73-b873-6717-fe45-a854dbdd5476@huaweicloud.com>
Date:   Tue, 21 Feb 2023 09:47:03 +0800
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Subject: Re: [PATCH bpf-next v2] riscv, bpf: Add kfunc support for RV64
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To:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        bpf@vger.kernel.org, linux-riscv@lists.infradead.org,
        linux-kernel@vger.kernel.org
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Pu Lehui <pulehui@huawei.com>
References: <20230220083203.2988238-1-pulehui@huaweicloud.com>
 <874jrg76dc.fsf@all.your.base.are.belong.to.us>
From:   Pu Lehui <pulehui@huaweicloud.com>
In-Reply-To: <874jrg76dc.fsf@all.your.base.are.belong.to.us>
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On 2023/2/20 22:34, Björn Töpel wrote:
> Pu Lehui <pulehui@huaweicloud.com> writes:
> 
>> From: Pu Lehui <pulehui@huawei.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