linux-kernel - Re: [PATCH bpf-next v2 1/1] bpf, arm64: support exceptions

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Date:   Sun, 17 Sep 2023 03:50:02 +0200
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 v2 1/1] bpf, arm64: support exceptions

On Sun, 17 Sept 2023 at 02:01, Puranjay Mohan <puranjay12@...il.com> wrote:
>
> Implement arch_bpf_stack_walk() for the ARM64 JIT. This will be used
> by bpf_throw() to unwind till the program marked as exception boundary and
> run the callback with the stack of the main program.
>
> The prologue generation code has been modified to make the callback
> program use the stack of the program marked as exception boundary where
> callee-saved registers are already pushed.
>
> As the bpf_throw function never returns, if it clobbers any callee-saved
> registers, they would remain clobbered. So, the prologue of the
> exception-boundary program is modified to push R23 and R24 as well,
> which the callback will then recover in its epilogue.
>
> The Procedure Call Standard for the Arm 64-bit Architecture[1] states
> that registers r19 to r28 should be saved by the callee. BPF programs on
> ARM64 already save all callee-saved registers except r23 and r24. This
> patch adds an instruction in prologue of the  program to save these
> two registers and another instruction in the epilogue to recover them.
>
> These extra instructions are only added if bpf_throw() used. Otherwise
> the emitted prologue/epilogue remains unchanged.
>
> [1] https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst
>
> Signed-off-by: Puranjay Mohan <puranjay12@...il.com>
> ---

We need reviews from arm64 JIT experts, but otherwise, given we've
discussed this offline as well:

Acked-by: Kumar Kartikeya Dwivedi <memxor@...il.com>