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

[Antoine Martin]

On 03/22/2010 02:17 AM, Ingo Molnar wrote:
> * Anthony Liguori<anthony@xxxxxxxxxxxxx>  wrote:
>    
> > On 03/19/2010 03:53 AM, Ingo Molnar wrote:
> >      
> > > * Avi Kivity<avi@xxxxxxxxxx>   wrote:
> > >        
> > > > > There were two negative reactions immediately, both showed a fundamental
> > > > > server versus desktop bias:
> > > > >
> > > > >   - you did not accept that the most important usecase is when there is a
> > > > >     single guest running.
> > > > >            
> > > > Well, it isn't.
> > > >          
> > > Erm, my usability points are _doubly_ true when there are multiple guests ...
> > >
> > > The inconvenience of having to type:
> > >
> > >    perf kvm --host --guest --guestkallsyms=/home/ymzhang/guest/kallsyms \
> > >    --guestmodules=/home/ymzhang/guest/modules top
> > >
> > > is very obvious even with a single guest. Now multiply that by more guests ...
> > >        
> > If you want to improve this, you need to do the following:
> >
> > 1) Add a userspace daemon that uses vmchannel that runs in the guest and can
> >     fetch kallsyms and arbitrary modules.  If that daemon lives in
> >     tools/perf, that's fine.
> >      
> Adding any new daemon to an existing guest is a deployment and usability
> nightmare.
>    
Absolutely. In most cases it is not desirable, and you'll find that in a 
lot of cases it is not even possible - for non-technical reasons.
One of the main benefits of virtualization is the ability to manage and 
see things from the outside.
> The basic rule of good instrumentation is to be transparent. The moment we
> have to modify the user-space of a guest just to monitor it, the purpose of
> transparent instrumentation is defeated.
>    
Not to mention Heisenbugs and interference.

Cheers
Antoine

> That was one of the fundamental usability mistakes of Oprofile.
>
> There is no 'perf' daemon - all the perf functionality is _built in_, and for
> very good reasons. It is one of the main reasons for perf's success as well.
>
> Now Qemu is trying to repeat that stupid mistake ...
>
> So please either suggest a different transparent solution that is technically
> better than the one i suggested, or you should concede the point really.
>
> Please try think with the heads of our users and developers and dont suggest
> some weird ivory-tower design that is totally impractical ...
>
> And no, you have to code none of this, we'll do all the coding. The only thing
> we are asking is for you to not stand in the way of good usability ...
>
> Thanks,
>
> 	Ingo
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