Re: [Xen-users] difference between Xen and Dom0 kernel

[Tapas Mishra <tapas@xxxxxxxxxxxx>]

Thanks for your reply .
>From your reply what I inferred is that to be able to use
virtualization from XEN
I need to install Hypervisor
Xen tools and then I can start working on like installing guest
operating systems
in the installed hypervisor.
Say for example I am having a laptop with Ubuntu installed.
So I need to install Xen Hypervisor on it and
then install a Dom0 kernel assuming that ( I downloaded the source
hypervisor without linux-2.6.18 )
from http://www.xen.org/products/xen_source.html
then I need to compile one Dom0 kernel as per the instructions given
http://wiki.xensource.com/xenwiki/XenParavirtOps
What does the following command do ?
git clone git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git
linux-2.6-xen

Or I need to download a kernel from www.kernel.org and compile it so
that I can use it as Dom0 why can I not use the kernel present on my
laptop in the installed Ubuntu as Dom0 and after installing hypervisor
start working on it.
To be able to install hypervisor what do I need to do.


On Thu, Nov 12, 2009 at 2:45 PM, Fajar A. Nugraha <fajar@xxxxxxxxx> wrote:
> On Thu, Nov 12, 2009 at 3:47 PM, Tapas Mishra <tapas@xxxxxxxxxxxx> wrote:
>> I want to know what is the difference between Xen and a Dom 0 kernel.
>> Then using Xen hypervisor and xen tools what are they.
>
> A hypervisor "is a piece of software/hardware platform-virtualization
> software that allows multiple operating systems to run on a host
> computer concurrently". These "mutiple operating systems" are called
> domains (dom) in Xen terminology. In an over-simplified term, the
> hypervisor will take care of allocating resources (CPU, memory, etc.)
> between the multiple running OS. The xen hypervisor is usually
> installed as /boot/xen*.gz on Linux dom0s. To control the xen
> hypervisor you need xen userland tools (example: /usr/sbin/xm).
>
> dom0 is special priviledged domain which has access to physical I/O
> resources (storage, NIC) and makes it available for other domains
> (domUs). It requires a special kernel (dom0 kernel) that supports Xen
> and have the necessary drivers to access physical resources.
>
> PV domUs are special kind of domUs which requires domU kernel to be
> modified to support Xen, thus providing minimum performance penalty.
> dom0 kernel can also be used as PV domU kernel.
>
> HVM domU is a kind of domU that doesn't require kernel modification.
> Most modern OS (e.g. Windows XP, Linux, Solaris 10) can run inside HVM
> domU using the same installer that you use to run it on bare metal.
> Usually it has higher performance penalty compared to PV domU.
>
> Note that Linux KVM takes a different approach from Xen : the kernel
> of priviledged domain also acts as hypervisor.
> Also the future goal is to integrate Xen support into upstream so that
> in the future you can have one kernel that can run on bare metal and
> Xen dom0/domU.
>
> --
> Fajar
>

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