Re: [PATCH] Enhance perf to collect KVM guest os statistics from host side

Anthony Liguori
2010-03-16T17:34:18Z

On 03/16/2010 10:52 AM, Ingo Molnar wrote:
> You are quite mistaken: KVM isnt really a 'random unprivileged application' in
> this context, it is clearly an extension of system/kernel services.
>
> ( Which can be seen from the simple fact that what started the discussion was
> 'how do we get /proc/kallsyms from the guest'. I.e. an extension of the
> existing host-space /proc/kallsyms was desired. )
>

Random tools (like perf) should not be able to do what you describe.
It's a security nightmare.

If it's desirable to have /proc/kallsyms available, we can expose an
interface in QEMU to provide that. That can then be plumbed through
libvirt and QMP.

Then a management tool can use libvirt or QMP to obtain that information
and interact with the kernel appropriately.

> In that sense the most natural 'extension' would be the solution i mentioned a
> week or two ago: to have a (read only) mount of all guest filesystems, plus a
> channel for profiling/tracing data. That would make symbol parsing easier and
> it's what extends the existing 'host space' abstraction in the most natural
> way.
>
> ( It doesnt even have to be done via the kernel - Qemu could implement that
> via FUSE for example. )
>

No way. The guest has sensitive data and exposing it widely on the host
is a bad thing to do. It's a bad interface. We can expose specific
information about guests but only through our existing channels which
are validated through a security infrastructure.

Ultimately, your goal is to keep perf a simple tool with little
dependencies. But practically speaking, if you want to add features to
it, it's going to have to interact with other subsystems in the
appropriate way. That means, it's going to need to interact with
libvirt or QMP.

If you want all applications to expose their data via synthetic file
systems, then there's always plan9 :-)

Regards,

Anthony Liguori