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

[Vince Weaver]

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