Message-ID: <521DFF1D.4020908@linux.vnet.ibm.com>
Date: Wed, 28 Aug 2013 15:46:05 +0200
From: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
MIME-Version: 1.0
To: Frantisek Hrbata <fhrbata@redhat.com>
CC: linux-kernel@vger.kernel.org, jstancek@redhat.com, keescook@chromium.org,
        rusty@rustcorp.com.au, linux-arch@vger.kernel.org, arnd@arndb.de,
        mgahagan@redhat.com, agospoda@redhat.com,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 3/4] gcov: compile specific gcov implementation based
 on gcc version
References: <1377247176-13537-1-git-send-email-fhrbata@redhat.com> <1377247176-13537-4-git-send-email-fhrbata@redhat.com> <52177C87.4090003@linux.vnet.ibm.com> <52177DE8.6090704@linux.vnet.ibm.com> <20130824194413.GB2365@localhost.localdomain> <521B62AF.2030408@linux.vnet.ibm.com> <20130827133439.GB2418@localhost.localdomain>
In-Reply-To: <20130827133439.GB2418@localhost.localdomain>
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On 27.08.2013 15:34, Frantisek Hrbata wrote:
> On Mon, Aug 26, 2013 at 04:14:07PM +0200, Peter Oberparleiter wrote:
>> On 24.08.2013 21:44, Frantisek Hrbata wrote:
>>> On Fri, Aug 23, 2013 at 05:21:12PM +0200, Peter Oberparleiter wrote:
>>>> On 23.08.2013 17:15, Peter Oberparleiter wrote:
>>>>> On 23.08.2013 10:39, Frantisek Hrbata wrote:
>>>>>> Compile the correct gcov implementation file for a specific gcc version. In
>>>>>> the future, if another file is added, the conditions will need to be somehow
>>>>>> adjusted to if-elif-else case, but at this point the simple cc-ifversion should
>>>>>> be enough.
>>>>
>>>> As promised, I'm also adding the patch that makes the format-specific part
>>>> of gcov-kernel a loadable kernel module:
>>>>
>>>> ---
>>>> kernel: gcov: make format-specific code loadable
>>>>
>>>> Turn the format-specific part of gcov-kernel into a loadable kernel
>>>> module. This enables the use of gcov-kernel with kernel modules
>>>> that were compiled with a version of GCC that produces a different
>>>> gcov format when compared to the version of GCC that was used to
>>>> compile the kernel.
>>>
>>> If I understand it correctly, this would mean that you will be able to use only
>>> one implementation of gcov format at the time. Meaning you will be able to get
>>> coverage data for module, but not for kernel if it was compiled with different
>>> gcc(gcda format). This is probably ok if you work only on your module, but I'm
>>> not sure this is generally the right approach. In this case I would probably
>>> rather see some support for more gcov formats at the same time(e.g. set of
>>> callback operations per gcov version). Again I'm probably missing something, but
>>> I still cannot see reason why to add such feature. If you want gcov support just
>>> compile your kernel and modules with the same gcc version(gcda format). But if
>>> this is really needed maybe it would be better to consider some parallel support
>>> for more gcov formats based on the gcov_info version.
>>
>> The callback approach has other drawbacks (see previous mail).
> 
> Agreed, I did not realized these problems for the first time when I was thinking
> about the callback approach.
> 	
>>
>>> Would it be possible to add support for the modified gcc 4.7 gcov format and
>>> deal with this later? I can incorporate your changes: iter to use buffer,
>>> .init_array for modules and possibility to explicitly select the gcda format.
>>> In this case we will have at least the basic support in kernel. This is just me
>>> thinking out loud.
>>
>> I think that's an approach I can live with. Maybe the need for a multi-version
>> support will surface again later in a more refined form, but until then there
>> should be no reason to delay base GCC 4.7 support any further.
> 
> Great. I can incorporate the changes you proposed and post V2. Or do you prefer
> to post it by your self? Based on the info and patches you already provided I
> guess you already have something ready. Simply what suits you best :).

If it's not too much hassle, I'd prefer that you post V2 of your patches. Also
I guess it would make sense to put Andrew Morton on CC since the initial
gcov-patches were picked up by him.

-- 
Peter Oberparleiter
Linux on System z Development - IBM Germany

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