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

[Avi Kivity <avi@redhat.com>]

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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