linux-kernel - Re: [Qemu-devel] [PATCH] KVM: Add wrapper script around QEMU to test kernels

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Date:	Mon, 7 Nov 2011 12:03:37 -0500 (EST)
From:	Vince Weaver <vince@...ter.net>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	Ted Ts'o <tytso@....edu>, Pekka Enberg <penberg@...nel.org>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Avi Kivity <avi@...hat.com>,
	"kvm@...r.kernel.org list" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
	qemu-devel Developers <qemu-devel@...gnu.org>,
	Alexander Graf <agraf@...e.de>,
	Blue Swirl <blauwirbel@...il.com>,
	Américo Wang <xiyou.wangcong@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [Qemu-devel] [PATCH] KVM: Add wrapper script around QEMU to test
 kernels

On Mon, 7 Nov 2011, Pekka Enberg wrote:

> I've never heard ABI incompatibility used as an argument for perf. Ingo?

Never overtly.  They're too clever for that.

In any case, as a primary developer of a library (PAPI) that uses the 
perf_events ABI I have to say that having perf in the kernel has been a 
*major* pain for us.

Unlike the perf developers, we *do* have to maintain backwards 
compatability.  And we have a lot of nasty code in PAPI to handle this.
Entirely because the perf_events ABI is not stable.  It's mostly stable, 
but there are enough regressions to be a pain.

It's problem enough that there's no way to know what version of the 
perf_event abi you are running against and we have to guess based on 
kernel version.  This gets "fun" because all of the vendors have 
backported seemingly random chunks of perf_event code to their older 
kernels.

And it often does seem as the perf developers don't care when something 
breaks in perf_events if it doesn't affect perf users.

For example, the new NMI watchdog severely breaks perf_event event 
allocation if you are using FORMAT_GROUP.  perf doesn't use this though, 
so none of the kernel developers seem to care.  And unless I can quickly 
come up with a patch as an outsider, a few kernel versions will go by and 
the kernel devs will declare "well it was broken so long, now we don't 
have to fix it".  Fun.

Vince
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