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

Avi Kivity
2010-03-16T12:41:14Z

On 03/16/2010 02:29 PM, Ingo Molnar wrote:
> * Avi Kivity<avi@redhat.com> wrote:
>
>
>> On 03/16/2010 01:25 PM, Ingo Molnar wrote:
>>
>>>
>>>> I haven't followed vmchannel closely, but I think it is. vmchannel is
>>>> terminated in qemu on the host side, not in the host kernel. So perf would
>>>> need to connect to qemu.
>>>>
>>> Hm, that sounds rather messy if we want to use it to basically expose kernel
>>> functionality in a guest/host unified way. Is the qemu process discoverable in
>>> some secure way?
>>>
>> We know its pid.
>>
> How do i get a list of all 'guest instance PIDs',

Libvirt manages all qemus, but this should be implemented independently
of libvirt.

> and what is the way to talk
> to Qemu?
>

In general qemu exposes communication channels (such as the monitor) as
tcp connections, unix-domain sockets, stdio, etc. It's very flexible.

>>> Can we trust it?
>>>
>> No choice, it contains the guest address space.
>>
> I mean, i can trust a kernel service and i can trust /proc/kallsyms.
>
> Can perf trust a random process claiming to be Qemu? What's the trust
> mechanism here?
>

Obviously you can't trust anything you get from a guest, no matter how
you get it.

How do you trust a userspace program's symbols? you don't. How do you
get them? they're on a well-known location.

>>> Is there some proper tooling available to do it, or do we have to push it
>>> through 2-3 packages to get such a useful feature done?
>>>
>> libvirt manages qemu processes, but I don't think this should go through
>> libvirt. qemu can do this directly by opening a unix domain socket in a
>> well-known place.
>>
> So Qemu has never run into such problems before?
>
> ( Sounds weird - i think Qemu configuration itself should be done via a
> unix domain socket driven configuration protocol as well. )
>

That's exactly what happens. You invoke qemu with -monitor
unix:blah,server (or -qmp for a machine-readable format) and have your
management application connect to that. You can redirect guest serial
ports, console, parallel port, etc. to unix-domain or tcp sockets.
vmchannel is an extension of that mechanism.

>>> ( That is the general thought process how many cross-discipline useful
>>> desktop/server features hit the bit bucket before having had any chance of
>>> being vetted by users, and why Linux sucks so much when it comes to feature
>>> integration and application usability. )
>>>
>> You can't solve everything in the kernel, even with a well populated tools/.
>>
> Certainly not, but this is a technical problem in the kernel's domain, so it's
> a fair (and natural) expectation to be able to solve this within the kernel
> project.
>

Someone writing perf-gui outside the kernel would have the same
problems, no?

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