Re: [PATCH v7 2/3] arm64: implement ftrace with regs

From: Julien Thierry <julien.thierry@arm.com>
To: Torsten Duwe <duwe@lst.de>
Cc: 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>,
	"Singh, Balbir" <bsingharora@gmail.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, live-patching@vger.kernel.org
Subject: Re: [PATCH v7 2/3] arm64: implement ftrace with regs
Date: Tue, 22 Jan 2019 13:49:50 +0000	[thread overview]
Message-ID: <72a3bb44-60d6-daba-0c9b-099eab2d05e7@arm.com> (raw)
In-Reply-To: <20190122132832.GB16778@lst.de>

On 22/01/2019 13:28, Torsten Duwe wrote:
> On Tue, Jan 22, 2019 at 10:18:17AM +0000, Julien Thierry wrote:
>> Hi Torsten,
>>
>> A few suggestions below.
>>
>>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>>> +#define ARCH_SUPPORTS_FTRACE_OPS 1
>>> +#define REC_IP_BRANCH_OFFSET AARCH64_INSN_SIZE
>>> +/* All we need is some magic value. Simply use "_mCount:" */
>>
>> Nit: Should the casing be "_mcount" ?
> 
> No! The string makes it clear what it's supposed to be and the peculiar
> casing makes it unique and leaves no doubt were it came from. So whenever
> you see this register value in a crash dump you don't have to wonder about
> weird linkage errors, as it surely did not originate from a symtab.
> 

Right, I had missed the point that the value below is actually the
binary representation of that string. Things make more sense now, thanks.

>>> +#define MCOUNT_ADDR		(0x5f6d436f756e743a)
>>> +#else
>>> +#define REC_IP_BRANCH_OFFSET 0
>>> +#define MCOUNT_ADDR		((unsigned long)_mcount)
>>> +#endif
>>> +

-- 
Julien Thierry