Re: kvmtool tree (Was: Re: [patch] config: fix make kvmconfig)

From: Ingo Molnar
Date: Mon Feb 11 2013 - 12:28:15 EST

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, Feb 11, 2013 at 4:26 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > If you are asking whether it is critical for the kernel
> > project to have tools/kvm/ integrated then it isn't. The
> > kernel will live just fine without it, even if that decision
> > is a mistake.
> You go on to explain how this helps kvmtool, and quite
> frankly, I DO NOT CARE.

Me and Pekka also listed several examples of how it already
helped the kernel, despite it only being partially present in
some kernel repos:

- Pekka listed new virtio drivers that were done via tools/kvm.

- Pekka listed ARM KVM support which was written/prototyped
using tools/kvm.

- There's over a dozen bugfixes in your kernel which were found
via syscall fuzzing built into tools/kvm. (I can dig them all
out if you want.)

- There are several fixes in the kernel side KVM subsystem
itself that were unearthed via tools/kvm.

- I showed how it helped the kernel by creating user-space
lockdep: code used in more situations means more exposure,
more bugfixes and more contributors. (It also allowed
immediate lockdep coverage for all the user-space mutexes
that tools/perf/ itself uses.)

Those were all real benefits to the kernel which are upstream or
almost upstream today.

This tool alone generated a *more* versatile number of
improvements to the kernel than the majority of filesystems and
the majority of drivers in the Linux kernel tree today has ever

How on earth can anyone, against all that evidence, still claim
that it's a net minus?

> Everything you talk about is about helping your work, by
> making the kernel maintenance be more. The fact that you want
> to use kernel infrastructure in kvmtool is a great example:
> you may think it's a great thing, but for the kernel it's just
> extra work, and extra layers of abstraction etc etc.

At least in the case of lockdep, the kernel side change enabling
it was a trivial:

kernel/lockdep.c | 4 ++++
1 file changed, 4 insertions(+)

... but we could probably make even that interaction go away and
make it exactly zero, and push all the changes on to the
liblockdep side.

Anyway, should I consider user space lockdep NAK-ed too in this
form, before we put any more work into it?

> And then you make it clear that you haven't even *bothered* to
> try to make it a separate project.

Just like back in the days you haven't even bothered to make
Linux a proper GNU project, and for good reasons. Nor have I
ever tried to write scheduler code for another OS.

Some expensive, disruptive things you don't "try" because you
disagree with the model, on what you think to be valid grounds.
You make a judgement call, you take your chances, and you see
whether it works.

> Sorry, but with that kind of approach, I get less and less
> interested. I think this whole tying together is a big
> mistake. It encourages linkages that simply shouldn't be
> there.
> And no, perf is not the perfect counter-example. With perf,.
> the linkages made sense! There's supposed to be deep linkages
> to profiling and event counting. There is ABSOLUTELY NOT
> supposed to be deep linkages with virtualization. Quite the
> reverse.

I'm *really* surprised that you say that.

perf interfaces to the Linux kernel in a very similar way to how
tools/kvm interfaces to the Linux kernel: both use (very) Linux
specific system calls to run on a host system running the Linux

Neither tool will in the foreseeable future execute on any
other, non-Linux host kernel.

( Running non-Linux guest OS's is an entirely different matter
and there's no fundamental restriction there. )

tools/perf/ is not in the kernel because it interfaces with the
kernel in nasty, incestous ways.

It is in the kernel mainly because that's the contribution model
that turned out to work well for both the kernel and the tooling

I initially made the user-space bits of perf a separate project.
I didn't run it in that form for a long time, but I got
essentially zero contributions. The moment it went into a silly
directory in Documentation/perf_counters/, contributions started
trickling in. Today it's fully embraced by the kernel side
subsystem and allows very efficient interface enhancements on
the kernel and tooling side, in lock-step - benefiting both
sides significantly.

> And no, I don't want to maintain the mess that is both.
> There's just no gain, and lots of potential pain.

I disagree, and I don't see the validity of most of your
arguments, but it's obviously your call.


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