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SubjectRe: [RFC] Unify KVM kernel-space and user-space code into a single project

* Avi Kivity <> wrote:

> On 03/21/2010 09:17 PM, Ingo Molnar wrote:
> >
> > Adding any new daemon to an existing guest is a deployment and usability
> > nightmare.
> The logical conclusion of that is that everything should be built into the
> kernel. [...]

Only if you apply it as a totalitarian rule.

Furthermore, the logical conclusion of _your_ line of argument (applied in a
totalitarian manner) is that 'nothing should be built into the kernel'.

I.e. you are arguing for microkernel Linux, while you see me as arguing for a
monolithic kernel.

Reality is that we are somewhere inbetween, we are neither black nor white:
it's shades of grey.

If we want to do a good job with all this then we observe subsystems, we see
how they relate to the physical world and decide about how to shape them. We
identify long-term changes and re-design modularization boundaries in
hindsight - when we got them wrong initially. We dont try to rationalize the

Lets see one example of that thought process in action: Oprofile.

We saw that the modularization of oprofile was a total nightmare: a separate
kernel-space and a separate user-space component, which was in constant
version friction. The ABI between them was stiffling: it was hard to change it
(you needed to trickle that through the tool as well which was on a different
release schedule, etc.e tc.)

The result was sucky usability that never went beyond some basic 'you can do
profiling' threshold. The subsystem worked well within that design box, and it
was worked on by highly competent people - but it was still far, far away from
the potential it could have achieved.

So we observed those problems and decided to do something about it:

- We unified the two parts into a single maintenance domain. There's
the kernel-side in kernel/perf_event.c and arch/*/*/perf_event.c,
plus the user-side in tools/perf/. The two are connected by a very
flexible, forwards and backwards compatible ABI.

- We moved much more code into the kernel, realizing that transparent
and robust instrumentation should be offered instead of punting
abstractions into user-space (which is in a disadvantaged position
to implement system-wide abstractions).

- We created a no-bullsh*t approach to usability. perf is by no means
perfect, but it's written by developers for developers and if you report a
bug to us we'll act on it before anything else. Furthermore the kernel
developers do the user-space coding as well, so there's no chinese
wall separating them. Kernel-space becomes aware of the intricacies of
user-space and user-space developers become aware of the difficulties of
kernel-space as well. It's a good mix in our experience.

The thing is (and i doubt you are surprised that i say that), i see a similar
situation with KVM. The basic parameters are comparable to Oprofile: it has a
kernel-space component and a KVM-specific user-space. By all practical means
the two are one and the same, but are maintained as different projects.

I have followed KVM since its inception with great interest. I saw its good
initial design, i tried it early on and even wrote various patches for it. So
i care more about KVM than a random observer would, but this preference and
passion for KVM's good technical sides does not cloud my judgement when it
comes to its weaknesses.

In fact the weaknesses are far more important to identify and express
publicly, so i tend to concentrate on them. Dont take this as me blasting KVM,
we both know the many good aspects of KVM.

So, as i explained it earlier in greater detail the modularization of KVM into
a separate kernel-space and user-space component is one of its worst current
weaknesses, and it has become the main stiffling force in the way of a better
KVM experience to users.

That, IMO, is the 'weakest link' of KVM today and no matter how well the rest
of KVM gets improved those nice bits all get unfairly ignored when the user
cannot have a usable and good desktop experience and thinks that KVM is

I think you should think outside the initial design box you have created 4
years ago, you should consider iterating the model and you should consider the
alternative i suggested: move (or create) KVM tooling to tools/kvm/ and treat
it as a single project from there on.



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