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

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Message-ID: <CAADnVQKftCnTnvyGwwWZu73zeqP8f_acoyMjwSpE+tj2dN1irQ@mail.gmail.com>
Date: Thu, 1 Feb 2024 12:36:31 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Björn Töpel <bjorn@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>, 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

On Thu, Feb 1, 2024 at 9:30 AM Björn Töpel <bjorn@...nel.org> wrote:
>
> 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?

Not sure. arm64, s390, loongarch, x86 claim to support it.
For a long time we thought that it works on x86 until Leon came up
with a way to break it.