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

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Message-ID: <871q9w9jno.fsf@all.your.base.are.belong.to.us>
Date: Thu, 01 Feb 2024 18:30:35 +0100
From: Björn Töpel <bjorn@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>, Daniel Borkmann
 <daniel@...earbox.net>
Cc: Pu Lehui <pulehui@...weicloud.com>, bpf <bpf@...r.kernel.org>,
 linux-riscv <linux-riscv@...ts.infradead.org>, Network Development
 <netdev@...r.kernel.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>, Leon Hwang <hffilwlqm@...il.com>
Subject: Re: [PATCH bpf-next v3 0/4] Mixing bpf2bpf and tailcalls for RV64

Alexei Starovoitov <alexei.starovoitov@...il.com> writes:

> On Thu, Feb 1, 2024 at 2:56 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>>
>> > will be destroyed. So we implemented mixing bpf2bpf and tailcalls
>> > similar to x86_64, i.e. using a non-callee saved register to transfer
> ...
>> 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.
>
> Hold on, folks.
> I'm not sure it's such a code idea to support tailcalls from subprogs
> in risc-v.
> They're broken on x86-64 and so far several attempts to fix them
> were not successful.
> If we don't have a fix soon we will disable this feature completely
> in the verifier.
> In general tailcalling from subprogs is a niche use case.
> If there are users they should transition to tail call from main prog only.
>
> See
> https://lore.kernel.org/bpf/CAADnVQJ1szry9P00wweVDu4d0AQoM_49qT-_ueirvggAiCZrpw@mail.gmail.com/

Got it. ...and it's broken on arm64 as well?