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From: Amit Kachhap <amit.kachhap@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-kernel@vger.kernel.org, catalin.marinas@arm.com,
	deller@gmx.de, duwe@suse.de,
	James.Bottomley@HansenPartnership.com, james.morse@arm.com,
	jeyu@kernel.org, jpoimboe@redhat.com, jthierry@redhat.com,
	linux-parisc@vger.kernel.org, mingo@redhat.com,
	peterz@infradead.org, svens@stackframe.org,
	takahiro.akashi@linaro.org, will@kernel.org
Subject: Re: [PATCHv2 1/8] ftrace: add ftrace_init_nop()
Date: Tue, 5 Nov 2019 12:17:26 +0530
Message-ID: <8e68de1f-f961-752d-9c07-ce41ce624d35@arm.com> (raw)
In-Reply-To: <20191104133657.GE45140@lakrids.cambridge.arm.com>

Hi Mark,

On 11/4/19 7:06 PM, Mark Rutland wrote:
> On Sat, Nov 02, 2019 at 05:49:00PM +0530, Amit Daniel Kachhap wrote:
>> Hi Mark,
> 
> Hi Amit,
> 
> Thanks for the review!
>   
>> On 10/29/19 10:28 PM, Mark Rutland wrote:
>>> Architectures may need to perform special initialization of ftrace
>>> callsites, and today they do so by special-casing ftrace_make_nop() when
>>> the expected branch address is MCOUNT_ADDR. In some cases (e.g. for
>>> patchable-function-entry), we don't have an mcount-like symbol and don't
>> s/an mcount/a mcount.
>>> want a synthetic MCOUNT_ADDR, but we may need to perform some
>>> initialization of callsites.
>>>
>>> To make it possible to separate initialization from runtime
>>> modification, and to handle cases without an mcount-like symbol, this
>> Same as above.
> 
> Using 'an' in both of these cases is correct, even though 'mcount'
> starts with a consonant, since it's pronounced as-if it were 'emcount'.
> I will leave this as-is.
Ok sure. It makes sense.
> 
>>> patch adds an optional ftrace_init_nop() function that architectures can
>>> implement, which does not pass a branch address.
>>>
>>> Where an architecture does not provide ftrace_init_nop(), we will fall
>>> back to the existing behaviour of calling ftrace_make_nop() with
>>> MCOUNT_ADDR.
>>>
>>> At the same time, ftrace_code_disable() is renamed to
>>> ftrace_nop_initialize() to make it clearer that it is intended to
>>> intialize a callsite into a disabled state, and is not for disabling a
>>> callsite that has been runtime enabled. The kerneldoc description of rec
>>> arguments is updated to cover non-mcount callsites.
>>>
>>> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
>>> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>>> Cc: Ingo Molnar <mingo@redhat.com>
>>> Cc: Steven Rostedt <rostedt@goodmis.org>
>>> Cc: Torsten Duwe <duwe@suse.de>
>>> ---
>>>    include/linux/ftrace.h | 35 ++++++++++++++++++++++++++++++++---
>>>    kernel/trace/ftrace.c  |  6 +++---
>>>    2 files changed, 35 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
>>> index 8a8cb3c401b2..9867d90d635e 100644
>>> --- a/include/linux/ftrace.h
>>> +++ b/include/linux/ftrace.h
>>> @@ -499,7 +499,7 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
>>>    /**
>>>     * ftrace_make_nop - convert code into nop
>>>     * @mod: module structure if called by module load initialization
>>> - * @rec: the mcount call site record
>>> + * @rec: the call site record (e.g. mcount/fentry)
>>>     * @addr: the address that the call site should be calling
>>>     *
>>>     * This is a very sensitive operation and great care needs
>>> @@ -520,9 +520,38 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
>>>    extern int ftrace_make_nop(struct module *mod,
>>>    			   struct dyn_ftrace *rec, unsigned long addr);
>>> +
>>> +/**
>>> + * ftrace_init_nop - initialize a nop call site
>>> + * @mod: module structure if called by module load initialization
>>> + * @rec: the call site record (e.g. mcount/fentry)
>>> + *
>>> + * This is a very sensitive operation and great care needs
>>> + * to be taken by the arch.  The operation should carefully
>>> + * read the location, check to see if what is read is indeed
>>> + * what we expect it to be, and then on success of the compare,
>>> + * it should write to the location.
>>> + *
>>> + * The code segment at @rec->ip should contain the contents created by
>>> + * the compiler
>> Nit: Will it be better to write it as "@rec->ip should store the adjusted
>> ftrace entry address of the call site" or something like that.
> 
> This was the specific wording requested by Steve, and it's trying to
> describe the instructions at rec->ip, rather than the value of rec->ip,
> so I think it's better to leave this as-is.
ok Its fine this way too. Actually from the comment, I could not 
understand which one of the compiler contents this points to as in this 
case there are 2 nops.
> 
>>> + *
>>> + * Return must be:
>>> + *  0 on success
>>> + *  -EFAULT on error reading the location
>>> + *  -EINVAL on a failed compare of the contents
>>> + *  -EPERM  on error writing to the location
>>> + * Any other value will be considered a failure.
>>> + */
>>> +#ifndef ftrace_init_nop
>>> +static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
>>> +{
>>> +	return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
>>> +}
>>> +#endif
>>> +
>> Now that ftrace_init_nop is also an arch implemented function so it may be
>> added in Documentation/trace/ftrace-design.rst along with ftrace_make_nop.
>> In general also, adding some description about patchable-function-entry
>> in kernel Documentation will be useful.
> 
> I agree that would be a good thing, but as Steve suggests I will leave
> this for subsequent rework, as I think that also implies more
> rework/renaming in the core code (e.g. to more cleanly separate the
> notion of a callsite from mcount specifically).
ok.

Thanks,
Amit
> 
> Steve, if you'd like help with that (or review), I'd be happy to help.
> 
> Thanks,
> Mark.
> 

  reply index

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-29 16:58 [PATCHv2 0/8] arm64: ftrace cleanup + FTRACE_WITH_REGS Mark Rutland
2019-10-29 16:58 ` [PATCHv2 1/8] ftrace: add ftrace_init_nop() Mark Rutland
2019-10-30 15:00   ` Miroslav Benes
2019-11-02 12:19   ` Amit Daniel Kachhap
2019-11-04 13:11     ` Steven Rostedt
2019-11-05  6:59       ` Amit Kachhap
2019-11-04 13:36     ` Mark Rutland
2019-11-05  6:47       ` Amit Kachhap [this message]
2019-11-06 14:15         ` Mark Rutland
2019-11-07  4:40           ` Amit Kachhap
2019-11-04 13:16   ` Steven Rostedt
2019-11-04 13:38     ` Mark Rutland
2019-11-04 13:53       ` Steven Rostedt
2019-10-29 16:58 ` [PATCHv2 2/8] module/ftrace: handle patchable-function-entry Mark Rutland
2019-10-30 15:03   ` Torsten Duwe
2019-10-31  9:02     ` Mark Rutland
2019-10-31 11:42       ` Torsten Duwe
2019-10-31 13:00         ` Mark Rutland
2019-11-04 13:28           ` Steven Rostedt
2019-11-04 14:00             ` Mark Rutland
2019-11-04 13:25   ` Steven Rostedt
2019-11-04 15:51   ` Mark Rutland
2019-11-04 20:58     ` Helge Deller
2019-11-05  8:59   ` Miroslav Benes
2019-10-29 16:58 ` [PATCHv2 3/8] arm64: module: rework special section handling Mark Rutland
2019-10-30 15:25   ` Miroslav Benes
2019-10-29 16:58 ` [PATCHv2 4/8] arm64: module/ftrace: intialize PLT at load time Mark Rutland
2019-11-02 12:20   ` Amit Daniel Kachhap
2019-11-04 13:55     ` Mark Rutland
2019-10-29 16:58 ` [PATCHv2 5/8] arm64: insn: add encoder for MOV (register) Mark Rutland
2019-10-29 16:58 ` [PATCHv2 6/8] arm64: asm-offsets: add S_FP Mark Rutland
2019-10-29 16:58 ` [PATCHv2 7/8] arm64: implement ftrace with regs Mark Rutland
2019-11-02 12:21   ` Amit Daniel Kachhap
2019-11-04 13:51     ` Mark Rutland
     [not found]   ` <CANW9uyug8WKN2fR-FmcW-C_OO_OQ_AvukM+BR7wqiJ9eFQMO9Q@mail.gmail.com>
2019-11-15  7:45     ` Torsten Duwe
2019-11-15 13:59     ` Mark Rutland
2019-10-29 16:58 ` [PATCHv2 8/8] arm64: ftrace: minimize ifdeffery Mark Rutland
2019-10-30 17:02 ` [PATCHv2 0/8] arm64: ftrace cleanup + FTRACE_WITH_REGS Torsten Duwe
2019-10-31 17:16 ` Torsten Duwe
2019-11-01  9:08   ` Mark Rutland
2019-11-01 15:39 ` Sven Schnelle
2019-11-01 16:28   ` Mark Rutland
2019-11-02 12:12 ` Amit Daniel Kachhap
2019-11-04 12:56   ` Will Deacon
2019-11-04 13:03     ` Amit Kachhap
2019-11-04 14:04       ` Mark Rutland
2019-11-05  7:06         ` Amit Kachhap
2019-11-07 11:31 ` Catalin Marinas

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