linux-kernel - Re: [PATCH bpf-next v3 0/2] bpf, arm64: Support Exceptions

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Date: Tue, 20 Feb 2024 23:12:23 +0100
From: Kumar Kartikeya Dwivedi <memxor@...il.com>
To: Puranjay Mohan <puranjay12@...il.com>
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 <yonghong.song@...ux.dev>, 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>, Zi Shen Lim <zlim.lnx@...il.com>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, bpf@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 0/2] bpf, arm64: Support Exceptions

On Thu, 1 Feb 2024 at 13:52, Puranjay Mohan <puranjay12@...il.com> wrote:
>
> Changes in V2->V3:
> V2: https://lore.kernel.org/all/20230917000045.56377-1-puranjay12@gmail.com/
> - Use unwinder from stacktrace.c rather than open coding the unwind logic.
> - Fix a bug in the prologue related to BPF_FP (Xu Kuohai)
>
> Changes in V1->V2:
> V1: https://lore.kernel.org/all/20230912233942.6734-1-puranjay12@gmail.com/
> - Remove exceptions from DENYLIST.aarch64 as they are supported now.
>
> The base support for exceptions was merged with [1] and it was enabled for
> x86-64.
>
> This patch set enables the support on ARM64, all sefltests are passing:
>
> # ./test_progs -a exceptions
> #74/1    exceptions/exception_throw_always_1:OK
> [...]

I think this looks ok, would be nice if it received acks from arm64 experts.

If you have cycles to spare, please also look into
https://lore.kernel.org/bpf/20240201042109.1150490-1-memxor@gmail.com
and let me know how architecture independent the cleanup code right
now in the x86 JIT should be made so that we can do the
same for arm64 as well later. That would be required to complete the
support for cleanups.

I guess we just need bpf_frame_spilled_caller_reg_off to be arch
specific and lift the rest out into the BPF core.
I will make that change in v2 in any case.

Just a note but based on our off-list discussion for supporting this
stuff on riscv as well (where a lot of registers would have to be
saved on entry),
the hidden subprog trampoline could be a way to avoid that. They can
be pushed by this subprog before entry into bpf_throw, and since the
BPF program does not touch the extra callee-saved registers, they
should be what the kernel had originally upon entry into the BPF
program itself. The same could be done on arm64 and x86, but the
returns would be diminishing. It would be nice to quantify how much
this saves in terms of costs of pushing extra registers, before doing
this.