linux-kernel - Re: [PATCH bpf-next v1 0/4] Support bpf trampoline for RV64

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Message-ID: <87mt5ft2bx.fsf@all.your.base.are.belong.to.us>
Date:   Wed, 15 Feb 2023 15:45:06 +0100
From:   Björn Töpel <bjorn@...nel.org>
To:     Pu Lehui <pulehui@...weicloud.com>, bpf@...r.kernel.org,
        linux-riscv@...ts.infradead.org, netdev@...r.kernel.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>,
        Palmer Dabbelt <palmer@...belt.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Guo Ren <guoren@...nel.org>,
        Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>, Pu Lehui <pulehui@...wei.com>,
        Pu Lehui <pulehui@...weicloud.com>
Subject: Re: [PATCH bpf-next v1 0/4] Support bpf trampoline for RV64

Hi Lehui,

Pu Lehui <pulehui@...weicloud.com> writes:

> BPF trampoline is the critical infrastructure of the bpf
> subsystem, acting as a mediator between kernel functions
> and BPF programs. Numerous important features, such as
> using ebpf program for zero overhead kernel introspection,
> rely on this key component. We can't wait to support bpf
> trampoline on RV64. Since RV64 does not support ftrace
> direct call yet, the current RV64 bpf trampoline is only
> used in bpf context.

Thanks a lot for continuing this work. I agree with you that it's
valuable to have BPF trampoline support, even without proper direct call
support (we'll get there soon). The trampoline enables kfunc calls. On
that note; I don't see that you enable "bpf_jit_supports_kfunc_call()"
anywhere in the series.  With BPF trampoline support, the RISC-V BPF
finally can support kfunc calls!

I'd add the following to bpf_jit_comp64.c:

bool bpf_jit_supports_kfunc_call(void)
{
        return true;
}

:-)

I'll do a review ASAP.


Björn