linux-kernel - Re: [PATCH v5 17/17] powerpc64/bpf: Add support for bpf trampolines

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Message-ID: <CAADnVQL60XXW95tgwKn3kVgSQAN7gr1STy=APuO1xQD7mz-aXA@mail.gmail.com>
Date: Tue, 17 Sep 2024 09:50:33 +0200
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Hari Bathini <hbathini@...ux.ibm.com>
Cc: linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>, bpf <bpf@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>, 
	Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	"Naveen N. Rao" <naveen@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Daniel Borkmann <daniel@...earbox.net>, Masahiro Yamada <masahiroy@...nel.org>, 
	Nicholas Piggin <npiggin@...il.com>, Alexei Starovoitov <ast@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Andrii Nakryiko <andrii@...nel.org>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Vishal Chourasia <vishalc@...ux.ibm.com>, Mahesh J Salgaonkar <mahesh@...ux.ibm.com>, 
	Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v5 17/17] powerpc64/bpf: Add support for bpf trampolines

On Sun, Sep 15, 2024 at 10:58 PM Hari Bathini <hbathini@...ux.ibm.com> wrote:
>
> +
> +       /*
> +        * Generated stack layout:
> +        *
> +        * func prev back chain         [ back chain        ]
> +        *                              [                   ]
> +        * bpf prog redzone/tailcallcnt [ ...               ] 64 bytes (64-bit powerpc)
> +        *                              [                   ] --
...
> +
> +       /* Dummy frame size for proper unwind - includes 64-bytes red zone for 64-bit powerpc */
> +       bpf_dummy_frame_size = STACK_FRAME_MIN_SIZE + 64;

What is the goal of such a large "red zone" ?
The kernel stack is a limited resource.
Why reserve 64 bytes ?
tail call cnt can probably be optional as well.