linux-kernel - Re: [RFC PATCH 0/4] add support for gcov format introduced in gcc 4.7

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Message-ID: <52177AD7.1030709@linux.vnet.ibm.com>
Date:	Fri, 23 Aug 2013 17:08:07 +0200
From:	Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
To:	Frantisek Hrbata <fhrbata@...hat.com>
CC:	linux-kernel@...r.kernel.org, jstancek@...hat.com,
	keescook@...omium.org,
	Christophe Guillon <christophe.guillon@...com>,
	rusty@...tcorp.com.au, linux-arch@...r.kernel.org, arnd@...db.de,
	mgahagan@...hat.com, agospoda@...hat.com
Subject: Re: [RFC PATCH 0/4] add support for gcov format introduced in gcc
 4.7

On 23.08.2013 10:39, Frantisek Hrbata wrote:
> This is an attempt to bring support for modified gcov format in gcc 4.7 to
> the kernel. It tries to leverage the existing layout/abstraction, which was
> designed keeping in mind that the gcov format could change, but some changes had
> to be make. Mostly because the current model does not take into account that
> even the core gcov structures, like gcov_info, could change. One part that could
> be problematic is the addition of the .init_array section for constructors.

It appears that gcc 4.7 support for gcov-kernel is quite important to a
number of people, at least that is what I derive from the fact that I
now know of 3 people who've been working on this support separately from
each other: you, myself (I've been close to posting my own version to
LKML) and Christophe Guillon.

It's apparent now that I made a mistake delaying the discussion of the
effort for too long, but I think your posting the patches opens up a
good opportunity to combine the best of all previous efforts.

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).

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

This way, the gcov-format specific part of gcov-kernel could be replaced
when working with a different version GCC. I'll post the corresponding
patches as reply in another mail.

Back to your patches: I tested them and they work fine on s390x when
compiled with GCC 4.3.4 and 4.7.2. I'll provide some more specific
comments as replies to your patch-mails.

Regards,
  Peter Oberparleiter

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

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