[Fedora-xen] problem with virtual interface in DomU

Paul Wouters paul at xelerance.com
Tue Sep 16 16:42:11 UTC 2008

On Tue, 16 Sep 2008, Guillaume wrote:

>>> It is not an HVM "build" so, i tought virtualization extensions are
>>> useless  ? And i don't have intel-VT on my system.
>> That may be the entire problem.
>> My understanding is that the paravirtualized kernel needs those extensions
>> so that the supervisor calls do not have to go through the hypervisor.
> For me it wasn't =) !
> For my knowledge, HVM extension was needed by xen only for
> fully-virtualized OS. (like Microsoft OS and other closed source OS ;
> because no modification can't be made to their kernel to make them
> aware of "xen".). But maybe i'm wrong !
> And if you're right, it will explain why networking don't work (and
> will never on my system)
>    --> but keep in mind that xen exist a "long time" before hvm
> extension do. And I thought that other people made networking works in
> their xen VM. So, I tought that i can get rid of this problem without
> hvm set on.
> Someone can light me more about it ?

I agree that things have gotten confusing in area of virtualisation.
This is what I understand:

- Xen hypervisor for para_virt guests does not need HVM. Problem here is
   that Fedora 8 is the last release to support this setup on x86_64, though
   work is in progress to add this support to Fedora 9/10.
   Para_virt guests are booted via kernel= and rootfs images, or via pygrub,
   which is just a wrapper for grabbing kernel from bootable disk images.

- Qemu is a software emulator for various architectures including PC hardware.
   It requires no HVM instructions, but it can use them if they exist via
   the kernel "kvm" code. This is how Fedora9 does its VM's via the libvirt
   and virt-install. This does NOT not use or require a xen hypervisor.

- Xenner is a software emulation for the Xen hypervisor. It requires HVM
   because it uses the kernel "kvm" code. The idea behind Xenner is that
   you can run VM's based on kernel-xen kernels (eg migration from Fedora8)

These are the main virtualisation techniques used in Fedora. There are
others, such as VirtualBox/Vmx, lguest, uml, virtuoso, openvz, and many
more. See http://virt.kernelnewbies.org/TechComparison


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