Re: xen bootcamp (was: xen: was Challenge to you: Voice your concerns regarding systemd) upstream

[Reco <recoverym4n@gmail.com>]

 Hi.

On Tue, Sep 30, 2014 at 04:16:41PM -0400, Steve Litt wrote:
> On Tue, 30 Sep 2014 17:57:41 +0400
> Reco <recoverym4n@gmail.com> wrote:
> 
> > Ok, ok. We all got it already. S*stemd in Debian = bad. S*stemd in
> > Fedora = good. Fedora has no xen, hence = bad. Debian has xen, hence =
> > good.
> > 
> > Reco
> 
> Reco, help me understand something: I don't understand why it
> matters what distro you chose for your xen dom0, or why it makes a
> distro better to have a dom0 xen kernel...

Sure. Once upon a time there was Xen. Then Xen was bought by Citrix.
Since then there's Citrix's Xen (aka Xenserver), Red Hat's Xen (nixed in
favor of qemu-kvm), Novell's Xen (nixed by Attachmate, a subsidiary of M$
who bought Novell), Debian's Xen, Oracle's Xen (aka OracleVM) and
NetBSD's Xen. There're also a non-public versions of Xen, such as
Amazon's one.


> My understanding of xen is it sits between the metal and your
> guest operating system(s). My understanding is there is a Linux host
> operating system, but its sole purpose is support of xen, and that's
> all it does. Here's a list of operating systems with xen dom0 kernels:
> 
> http://wiki.xenproject.org/wiki/Dom0_Kernels_for_Xen
> 
> By the way, the preceding page states that Fedora, from v16 on, has a
> dom0 xen kernel.

Xen is a microkernel, and there's two major versions of such microkernel
- 3 and 4. Debian currently uses version 4, about the only one who uses
version 3 today is Oracle. Xen's microkernel is a free software.

But there's more to that as there's Xen's API made for controlling
domUs, in fact, there's two sets of Xen's API - xm (made by human beings
for human begins), and its successor xl (made by our friends from Mars
for robots). Again, Xen's API is a free software.

But using Xen's API directly is not the thing that Joe the average admin
would do. Hence, there're tools which use Xen's API:

- venerable xm toolset
- hipster xl toolset
- RedHat's pet libvirt
- that thingie they put into XenServer
- an abomination they put into OracleVM
- that secret thing they in at Amazon

Hence a choice of distribution depends on a Xen toolset of choosing and
a huge portion of a personal taste.


> To my way of thinking, the dom0 OS isn't really being used as an OS at
> all: It's just an enabler for xen, and to my way of thinking, that
> means it has way fewer challenges than a normal OS. So if it uses
> systemd, so what? It's not like you have to worry about running Gimp or
> Gnome or KDE or various authentication facilities directly on it: It's
> a vehicle for xen, no more, no less.

On a typical virtualization server - yes, dom0 is used for managing
domUs, and not much more.

But, if you need to run X - things are very different, as you use dom0
as a conventional PC. Because - it's *very* difficult to force Xen to
transfer control of your video card to domU. Possible, but very
difficult, and hardware specific.


> I went to a Xen talk, and as I remember the guy said Xen is developed
> and tested on Ubuntu, so that's your best bet. Yeah, Ubuntu will have
> systemd, but if it works for the xen people, it should work for us all.

That guy does not seem to know what he's talking about. Xen is developed
by Xen Project (xen.org), Citrix and Oracle (they like to say so, at
least). Ubuntu merely builds Xen from the source, as others do.


> If one absolutely wants to avoid systemd, NetBSD has a xen kernel
> available.

So they say. They also say that NetBSD was the first, and it is the most
portable of BSDs.
But the reality is that BSD people say you 'it runs on this platform'
that usually means they give you so called 'base system' and a
toolchain. And if you have *a lot* of free time, you can build any
software you like as long as it's ported.
Debian (I need to compare BSD with something, do I?) says that 'it runs
on this platform' only once most (90% IIRC) software from the main
archive is actually built *and* runs on said platform.
Summing it up - I don't know current status of Xen on NetBSD, and
whenever Xen is in the 'base system' or 'ports'.


> Unless I'm vastly misinformed, once your dom0 xen is installed, you can
> now install domU hosts of any type you want, with or without systemd,
> and use them to your heart's content.
> 
> Am I understanding the situation right?

Yes, that's correct. To run so called 'fully virtualized OS' (i.e. not
modified to work with Xen) you'll need to have Intel VT-d CPU flag (or
an appropriate AMD equivalent), but that's all that required.

Convincing domU to use a real hardware is entirely different story.

Reco