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

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Message-ID: <72a3bb44-60d6-daba-0c9b-099eab2d05e7@arm.com>
Date:   Tue, 22 Jan 2019 13:49:50 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Torsten Duwe <duwe@....de>
Cc:     Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Amit Daniel Kachhap <amit.kachhap@....com>,
        "Singh, Balbir" <bsingharora@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v7 2/3] arm64: implement ftrace with regs



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