linux-kernel - Re: [RFC] Unify KVM kernel-space and user-space code into a single project

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Message-ID: <4BA674F1.6070603@nagafix.co.uk>
Date:	Mon, 22 Mar 2010 02:35:13 +0700
From:	Antoine Martin <antoine@...afix.co.uk>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Anthony Liguori <anthony@...emonkey.ws>,
	Avi Kivity <avi@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Sheng Yang <sheng@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	oerg Roedel <joro@...tes.org>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>, ziteng.huang@...el.com,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Fr?d?ric Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC] Unify KVM kernel-space and user-space code into a single
 project

On 03/22/2010 02:17 AM, Ingo Molnar wrote:
> * Anthony Liguori<anthony@...emonkey.ws>  wrote:
>    
>> On 03/19/2010 03:53 AM, Ingo Molnar wrote:
>>      
>>> * Avi Kivity<avi@...hat.com>   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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>    

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