Re: [PATCH v8 4/5] arm64: implement ftrace with regs

From: Julien Thierry <julien.thierry@arm.com>
To: Torsten Duwe <duwe@lst.de>, Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: live-patching@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v8 4/5] arm64: implement ftrace with regs
Date: Wed, 13 Feb 2019 10:30:12 +0000	[thread overview]
Message-ID: <c6a24572-84d5-bf0f-f70c-8580a83834a1@arm.com> (raw)
In-Reply-To: <20190208151019.9E7A968DDD@newverein.lst.de>

Hi Torsten,

On 08/02/2019 15:10, Torsten Duwe wrote:
> Implement ftrace with regs, based on the new gcc flag
> -fpatchable-function-entry (=2)
> 
> Now that gcc8 added 2 NOPs at the beginning of each function, replace
> the first NOP thus generated with a quick LR saver (move it to scratch
> reg x9), so the 2nd replacement insn, the call to ftrace, does not
> clobber the value. Ftrace will then generate the standard stack
> frames.
> 
> Note that patchable-function-entry in GCC disables IPA-RA, which means
> ABI register calling conventions are obeyed and scratch registers such
> as x9 are available.
> 
> Introduce and handle an ftrace_regs_trampoline for module PLTs, right
> after ftrace_trampoline in an ftrace_trampolines[2] array, and double
> the size of the corresponding special section.
> 
> Signed-off-by: Torsten Duwe <duwe@suse.de>
> 
> ---
>  arch/arm64/include/asm/ftrace.h  |   16 ++++
>  arch/arm64/include/asm/module.h  |    3 
>  arch/arm64/kernel/entry-ftrace.S |  125 +++++++++++++++++++++++++++++++++++++--
>  arch/arm64/kernel/ftrace.c       |  117 ++++++++++++++++++++++++++++--------
>  arch/arm64/kernel/module-plts.c  |    3 
>  arch/arm64/kernel/module.c       |    2 
>  6 files changed, 231 insertions(+), 35 deletions(-)

[...]

> --- a/arch/arm64/include/asm/module.h
> +++ b/arch/arm64/include/asm/module.h
> @@ -32,7 +32,8 @@ struct mod_arch_specific {
>  	struct mod_plt_sec	init;
>  
>  	/* for CONFIG_DYNAMIC_FTRACE */
> -	struct plt_entry 	*ftrace_trampoline;
> +	struct plt_entry	*ftrace_trampolines;
> +#define MOD_ARCH_NR_FTRACE_TRAMPOLINES	2

Nit: I'd define this outside of the struct, just below
MODULE_ARCH_VERMAGIC, and use the prefix "MODULE_ARCH" instead of
"MOD_ARCH" as this seem to be already in use for a few other defines.

Otherwise things look good to me:

Reviewed-by: Julien Thierry <julien.thierry@arm.com>

Cheers,

-- 
Julien Thierry

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