netdev - Re: [PATCH bpf-next 4/4] riscv, bpf: Mixing bpf2bpf and tailcalls

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Date: Tue, 19 Sep 2023 12:50:43 +0100
From: Conor Dooley <conor@...nel.org>
To: Pu Lehui <pulehui@...wei.com>
Cc: Pu Lehui <pulehui@...weicloud.com>, bpf@...r.kernel.org,
	linux-riscv@...ts.infradead.org, netdev@...r.kernel.org,
	Björn Töpel <bjorn@...nel.org>,
	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>,
	Luke Nelson <luke.r.nels@...il.com>
Subject: Re: [PATCH bpf-next 4/4] riscv, bpf: Mixing bpf2bpf and tailcalls

On Tue, Sep 19, 2023 at 07:23:07PM +0800, Pu Lehui wrote:
> 
> 
> On 2023/9/19 18:04, Conor Dooley wrote:
> > On Tue, Sep 19, 2023 at 11:57:11AM +0800, Pu Lehui wrote:
> > > From: Pu Lehui <pulehui@...wei.com>
> > > 
> > > In the current RV64 JIT, if we just don't initialize the TCC in subprog,
> > > the TCC can be propagated from the parent process to the subprocess, but
> > > the TCC of the parent process cannot be restored when the subprocess
> > > exits. Since the RV64 TCC is initialized before saving the callee saved
> > > registers into the stack, we cannot use the callee saved register to
> > > pass the TCC, otherwise the original value of the callee saved register
> > > will be destroyed. So we implemented mixing bpf2bpf and tailcalls
> > > similar to x86_64, i.e. using a non-callee saved register to transfer
> > > the TCC between functions, and saving that register to the stack to
> > > protect the TCC value. At the same time, we also consider the scenario
> > > of mixing trampoline.
> > > 
> > > Tests test_bpf.ko and test_verifier have passed, as well as the relative
> > > testcases of test_progs*.
> > > 
> > > Signed-off-by: Pu Lehui <pulehui@...wei.com>
> > 
> > Breaks the build:
> > ../arch/riscv/net/bpf_jit_comp64.c:846:14: error: use of undeclared identifier 'BPF_TRAMP_F_TAIL_CALL_CTX'
> > 
> 
> Hi Conor,
> 
> BPF_TRAMP_F_TAIL_CALL_CTX rely on commit [0], and it has been merged into
> bpf-next tree.

I see. I did check the cover to see if there was anything relevant
there, like a link or base commit, but since there were neither I opted
to pass on the warning from the patchwork automation we have :)

Thanks & sorry for the noise on this one.

Thanks,
Conor.

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