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

Ingo Molnar
2010-03-22T19:22:59Z

* Anthony Liguori <anthony@codemonkey.ws> wrote:

> On 03/22/2010 12:34 PM, Ingo Molnar wrote:
> >* Avi Kivity<avi@redhat.com> wrote:
> >
> >>>>> - Easy default reference to guest instances, and a way for tools to
> >>>>> reference them symbolically as well in the multi-guest case. Preferably
> >>>>> something trustable and kernel-provided - not some indirect information
> >>>>> like a PID file created by libvirt-manager or so.
> >>>>Usually 'layering violation' is trotted out at such suggestions.
> >>>>[...]
> >>>That's weird, how can a feature request be a 'layering violation'?
> >>The 'something trustable and kernel-provided'. The kernel knows nothing
> >>about guest names.
> >The kernel certainly knows about other resources such as task names or network
> >interface names or tracepoint names. This is kernel design 101.
> >
> >>>If something that users find straightforward and usable is a layering
> >>>violation to you (such as easily being able to access their own files on
> >>>the host as well ...) then i think you need to revisit the definition of
> >>>that term instead of trying to fix the user.
> >>Here is the explanation, you left it quoted:
> >>
> >>>>[...] I don't like using the term, because sometimes the layers are
> >>>>incorrect and need to be violated. But it should be done explicitly, not
> >>>>as a shortcut for a minor feature (and profiling is a minor feature, most
> >>>>users will never use it, especially guest-from-host).
> >>>>
> >>>>The fact is we have well defined layers today, kvm virtualizes the cpu
> >>>>and memory, qemu emulates devices for a single guest, libvirt manages
> >>>>guests. We break this sometimes but there has to be a good reason. So
> >>>>perf needs to talk to libvirt if it wants names. Could be done via
> >>>>linking, or can be done using a pluging libvirt drops into perf.
> >This is really just the much-discredited microkernel approach for keeping
> >global enumeration data that should be kept by the kernel ...
> >
> >Lets look at the ${HOME}/.qemu/qmp/ enumeration method suggested by Anthony.
> >There's numerous ways that this can break:
> >
> > - Those special files can get corrupted, mis-setup, get out of sync, or can
> > be hard to discover.
> >
> > - The ${HOME}/.qemu/qmp/ solution suggested by Anthony has a very obvious
> > design flaw: it is per user. When i'm root i'd like to query _all_ current
> > guest images, not just the ones started by root. A system might not even
> > have a notion of '${HOME}'.
> >
> > - Apps might start KVM vcpu instances without adhering to the
> > ${HOME}/.qemu/qmp/ access method.
>
> Not all KVM vcpus are running operating systems.

But we want to allow developers to instrument all of them ...

> Transitive had a product that was using a KVM context to run their
> binary translator which allowed them full access to the host
> processes virtual address space range. In this case, there is no
> kernel and there are no devices.

And your point is that such vcpus should be excluded from profiling just
because they fall outside the Qemu/libvirt umbrella?

That is a ridiculous position.

Ingo