linux-kernel - Re: [RFC PATCH v4 12/17] powerpc64/ftrace: Move ftrace sequence out of line

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Message-Id: <D2PYW90LRVAY.3PCE9P3NE2NEB@gmail.com>
Date: Mon, 15 Jul 2024 18:25:10 +1000
From: "Nicholas Piggin" <npiggin@...il.com>
To: "Naveen N Rao" <naveen@...nel.org>, <linuxppc-dev@...ts.ozlabs.org>,
 <linux-trace-kernel@...r.kernel.org>, <bpf@...r.kernel.org>,
 <linux-kbuild@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Cc: "Michael Ellerman" <mpe@...erman.id.au>, "Christophe Leroy"
 <christophe.leroy@...roup.eu>, "Steven Rostedt" <rostedt@...dmis.org>,
 "Masami Hiramatsu" <mhiramat@...nel.org>, "Mark Rutland"
 <mark.rutland@....com>, "Alexei Starovoitov" <ast@...nel.org>, "Daniel
 Borkmann" <daniel@...earbox.net>, "Andrii Nakryiko" <andrii@...nel.org>,
 "Masahiro Yamada" <masahiroy@...nel.org>, "Hari Bathini"
 <hbathini@...ux.ibm.com>, "Mahesh Salgaonkar" <mahesh@...ux.ibm.com>,
 "Vishal Chourasia" <vishalc@...ux.ibm.com>
Subject: Re: [RFC PATCH v4 12/17] powerpc64/ftrace: Move ftrace sequence out
 of line

On Sun Jul 14, 2024 at 6:27 PM AEST, Naveen N Rao wrote:
> Function profile sequence on powerpc includes two instructions at the
> beginning of each function:
> 	mflr	r0
> 	bl	ftrace_caller
>
> The call to ftrace_caller() gets nop'ed out during kernel boot and is
> patched in when ftrace is enabled.
>
> Given the sequence, we cannot return from ftrace_caller with 'blr' as we
> need to keep LR and r0 intact. This results in link stack (return
> address predictor) imbalance when ftrace is enabled. To address that, we
> would like to use a three instruction sequence:
> 	mflr	r0
> 	bl	ftrace_caller
> 	mtlr	r0
>
> Further more, to support DYNAMIC_FTRACE_WITH_CALL_OPS, we need to
> reserve two instruction slots before the function. This results in a
> total of five instruction slots to be reserved for ftrace use on each
> function that is traced.
>
> Move the function profile sequence out-of-line to minimize its impact.
> To do this, we reserve a single nop at function entry using
> -fpatchable-function-entry=1 and add a pass on vmlinux.o to determine
> the total number of functions that can be traced. This is then used to
> generate a .S file reserving the appropriate amount of space for use as
> ftrace stubs, which is built and linked into vmlinux.

These are all going into .tramp.ftrace.text AFAIKS? Should that be
moved after some of the other text in the linker script then if it
could get quite large? sched and lock and other things should be
closer to the rest of text and hot code.

Thanks,
Nick