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

[Avi Kivity]

On 03/16/2010 02:29 PM, Ingo Molnar wrote:
> * Avi Kivity<avi@xxxxxxxxxx>  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?

--
error compiling committee.c: too many arguments to function

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