Message-ID: <521B428F.6020801@linux.vnet.ibm.com>
Date: Mon, 26 Aug 2013 13:57:03 +0200
From: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
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To: Frantisek Hrbata <fhrbata@redhat.com>
CC: linux-kernel@vger.kernel.org, jstancek@redhat.com, keescook@chromium.org,
        Christophe Guillon <christophe.guillon@st.com>, rusty@rustcorp.com.au,
        linux-arch@vger.kernel.org, arnd@arndb.de, mgahagan@redhat.com,
        agospoda@redhat.com
Subject: Re: [RFC PATCH 0/4] add support for gcov format introduced in gcc
 4.7
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On 23.08.2013 18:15, Frantisek Hrbata wrote:
> On Fri, Aug 23, 2013 at 05:08:07PM +0200, Peter Oberparleiter wrote:
>> Most of your code looks very familiar. There's one feature missing though
>> that Christophe brought up as a requirement: the ability for gcov-kernel
>> to cope with kernel modules being compiled with GCC versions implementing
>> a different gcov format (apparently this can happen in some embedded
>> setups).
> 
> Here follows quote from the gcc/gcov-io.h file
> 
> <quote>
> Coverage information is held in two files.  A notes file, which is
> generated by the compiler, and a data file, which is generated by
> the program under test.  Both files use a similar structure.  We do
> not attempt to make these files backwards compatible with previous
> versions, as you only need coverage information when developing a
> program.  We do hold version information, so that mismatches can be
> detected, and we use a format that allows tools to skip information
> they do not understand or are not interested in.
> </quote>
> 
> Also from my experience, I would expect that the gcov will be used during
> development, meaning re-compilation isn't a big problem here. I for sure can be
> missing something and I'm by no means saying it wouldn't be useful feature. Just
> that it would complite things a little bit.

Here's the info I got from Christophe when we last discussed this feature:
"The main issue is compilation of modules with a different
compiler from the kernel. It is a quite common situation in embedded
Linuxes, as modules can be published far after the core kernel, thus
for our use it is a good feature."

I'd say that if we can support that setup with manageable extra effort,
it would be worth it.

>> Christophe proposed run-time version checking and a file-ops type function
>> table which is chosen based on info->version. I found this approach
>> somewhat intrusive and this would also not have covered the case where a
>> new GCC versions was used to compile kernel modules for which the base
>> kernel has no support. I tried to solve this requirement by combining
>> two changes:
>>
>> 1) make the gcov-format generated by gcov-kernel compile-time configurable
>> 2) separate the gcov-format specific code into a loadable kernel module
> 
> So if I understand it correctly you would separate the input
> format(gcov_info) from the output(gcda files). Meaning no matter which gcc
> compiled the module you can select the gcda format. And even if the kernel does
> not know the new format you can simply compile a module supporting it, instead
> of the whole kernel.

Actually my proposal is far simpler:
- the gcov-kernel module supports one GCC format (both gcov_info as well as .gcda)
- the module can be recompiled with different format support
- it will create correct .gcda files for all object files compiled with a
  compatible compiler
- for other object files, either the resulting .gcda files are unusable or no
  .gcda file would be registered at all (based on info->version differences),
  though that would raise the question on how to handle older GCC versions
  into which the new gcov format code was integrated

> Can you please give me an example why this is needed? As I wrote I'm not that
> familiar with gcov and I'm probably missing something, but I do not understand
> why this(handling several versions at runtime, not only the one used by gcc
> during compilation) is useful(note my comment above about the gcov used during
> development).

See note above from Christophe: kernel and module are compiled by different parties
at different times using different GCC versions, and apparently the production kernel
has gcov support enabled or they are providing a separate test kernel for that.


Regards,
  Peter

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

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