linux-kernel - Re: [RFC PATCH] perfcounters: provide expansion room in the ABI

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Date:	Wed, 4 Mar 2009 11:49:45 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC PATCH] perfcounters: provide expansion room in the ABI


* Paul Mackerras <paulus@...ba.org> wrote:

> Impact: ABI change
> 
> This expands several fields in the perf_counter_hw_event 
> struct and adds a "flags" argument to the perf_counter_open 
> system call, in order that features can be added in future 
> without ABI changes.
> 
> In particular the record_type field is expanded to 64 bits, 
> and the space for flag bits has been expanded from 32 to 64 
> bits.
> 
> This also adds some new fields:
> 
> * read_format (64 bits) is intended to provide a way to specify what
>   userspace wants to get back when it does a read() on a simple
>   (non-interrupting) counter;
> 
> * exclude_idle (1 bit) provides a way for userspace to ask that events
>   that occur when the cpu is idle be excluded;
> 
> * extra_config_len will provide a way for userspace to supply an
>   arbitrary amount of extra machine-specific PMU configuration data
>   immediately following the perf_counter_hw_event struct, to allow
>   sophisticated users to program things such as instruction matching
>   CAMs and address range registers;
> 
> * __reserved_3 and __reserved_4 provide space for future expansion.
> 
> Signed-off-by: Paul Mackerras <paulus@...ba.org>
> ---
> This should provide plenty of room for future expansion, in fact maybe
> it's a little over the top.

Yeah - but i think it does not hurt to be defensive when it 
comes to ABI extensibility.

Your changes look very good to me.

> Ingo, if this looks OK, you can pull it from the rfc branch of my
> perfcounters.git tree at:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/paulus/perfcounters.git rfc
> 
>  include/linux/perf_counter.h |   12 +++++++++---
>  include/linux/syscalls.h     |    2 +-
>  kernel/perf_counter.c        |   10 +++++++---
>  3 files changed, 17 insertions(+), 7 deletions(-)

Pulled into tip:perfcounters/core, thanks Paul!

	Ingo
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