Subject: Re: "User mode NetBSD"
To: Ernesto Bascon <>
From: Evaldo Gardenali <>
List: netbsd-users
Date: 05/05/2007 18:25:58

1. Yes, feasible, but for what?

2. Not sure

"If both things are possible, a lot of possibilities can be done on it!"

Like what?

The main reason I see for UML to exist:

User Mode Linux has its reason because Linux has no good way of 
debugging the kernel other than that, unlike NetBSD that got ddb and 
serial port debugging options.

If you mean virtualizing stuff, my opinion follows:

Thanks to the excellent work of Manuel Bouyer and others (sorry, folks, 
but Manuel was the first name that came to my mind on that, and the one 
I see most often on the port-xen mails :) NetBSD runs on Xen for quite a 
while now, and now Xen can even boot unmodified OSes with appropriate 
hardware, so we can have a x86 box running many instances of pretty much 
any OS to our desires. For the ones that don't have such hardware, you 
can run a ton of Linux and NetBSD instances on a single box, with 

User Mode Linux is a resource hog, try googling around for the problems 
with context switching on UML which is a pain and keeps performance low. 
This is an inherent problem of User Mode kernels. UML should never be 
used for production, but still, a few people here and there think its 
the best thing since toilet paper.

If someone wants to virtualize their servers, normally they run on 
mainstream platforms like x86 and amd64 (Xen works on both, NetBSD/xen 
works on x86, hopefully soon on amd64), or high-end platforms which 
might have their own partitioning/virtualization software, just like IBM 
zSeries has z/VM partitioning/virtualization software. I doubt sincerely 
that someone will virtualize a NetBSD/vax, NetBSD/hpcmips, NetBSD/mac68k 
box for instance, so I see little use for such an effort.

For comparison, an old lecture I gave about Xen 2.0.x can be found here:
Please note that several things evolved since then, like Xen doesn't 
necessarily need a port to special arch anymore if you have the hardware.
The performance charts are labeled "L X V U" for Linux, Xen, Vmware, 

Kind Regards

Ernesto Bascon escreveu:
> HI everybody:
> I have no the needed skills to work on these areas, but:
> 1. Is it feasible to implement something like "User mode Linux" on
> NetBSD; id est, is it possible to create a NetBSD port to run on top
> of NetBSD? What would be the difficult things to do a port like that?
> 2. If that is possible, would be feasible to implement the NetBSD on
> top of NetBSD port 100% portable? I mean, if NetBSD has been ported to
> run on top of NetBSD syscalls instead of direct hardware access, would
> be possible implement this port ONCE and compile the same source with
> no modification on every supported platform?
> If both things are possible, a lot of possibilities can be done on it!
> Kind regards,
> Ernesto