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

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Date:	Mon, 22 Mar 2010 21:46:50 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Anthony Liguori <anthony@...emonkey.ws>,
	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>,
	Gregory Haskins <ghaskins@...ell.com>
Subject: Re: [RFC] Unify KVM kernel-space and user-space code into a single
 project


* Avi Kivity <avi@...hat.com> wrote:

> On 03/22/2010 09:27 PM, Ingo Molnar wrote:
> >
> >> If your position basically boils down to, we can't trust userspace
> >> and we can always trust the kernel, I want to eliminate any
> >> userspace path, then I can't really help you out.
> >
> > Why would you want to 'help me out'? I can tell a good solution from a bad 
> > one just fine.
> 
> You are basically making a kernel implementation a requirement, instead of 
> something that follows from the requirement.

No, i'm not.

> > You should instead read the long list of disadvantages above, invert them 
> > and list then as advantages for the kernel-based vcpu enumeration 
> > solution, apply common sense and go admit to yourself that indeed in this 
> > situation a kernel provided enumeration of vcpu contexts is the most 
> > robust solution.
> 
> Having qemu enumerate guests one way or another is not a good idea IMO since 
> it is focused on one guest and doesn't have a system-wide entity.  A 
> userspace system-wide entity will work just as well as kernel 
> implementation, without its disadvantages.

A system-wide user-space entity only solves one problem out of the 4 i listed, 
still leaving the other 3:

 - Those special files can get corrupted, mis-setup, get out of sync, or can
   be hard to discover.

 - Apps might start KVM vcpu instances without adhering to the
   system-wide access method.

 - There is no guarantee for the system-wide process to reply to a request -
   while the kernel can always guarantee an enumeration result. I dont want
   'perf kvm' to hang or misbehave just because the system-wide entity has 
   hung.

Really, i think i have to give up and not try to convince you guys about this 
anymore - i dont think you are arguing constructively anymore and i dont want 
yet another pointless flamewar about this.

Please consider 'perf kvm' scrapped indefinitely, due to lack of robust KVM 
instrumentation features: due to lack of robust+universal vcpu/guest 
enumeration and due to lack of robust+universal symbol access on the KVM side. 
It was a really promising feature IMO and i invested two days of arguments 
into it trying to find a workable solution, but it was not to be.

Thanks,

	Ingo
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