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

Antoine Martin
2010-03-21T19:35:25Z

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