Re: [Xen-users] Xen as Guest L1 - VirtIO front-end

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, May 27, 2015 at 9:47 PM, Alex Palesandro <palexster@xxxxxxxxx> wrote:
> Hi,
>
> I'm trying to virtualize Xen as L1 nested hypervisor. However, when I try to
> boot Dom0 linux kernel, the boot process hangs  (relevant output should be
> attached).
>
> I'm using KVM on ArchLinux with Linux 4.0.3 with qemu 2.3.0. The guest
> hypervisor is Xen 4.5 with Ubuntu 14.10 (Linux 3.16) as Dom0.
> Looking at the log, I'm guessing that the problem concerns VirtIO. Any Idea
> for a possible workaround? Is it possible to use the VirtIO front-end in
> Dom0?

Have you tried disabling virtio (i.e., using only emulated devices)
and seeing if that works?  That would give you a pretty clear "smoking
gun" as it were.

I don't know much about virtio.  But I wouldn't be surprised if the
virtio code assumes that it knows how to make hypercalls and talk to
the hypervisor.  But in this case, dom0 is technically an L2 guest (a
VM running on L1 Xen running on L0 KVM); in which case some
"hypervisor interactions" will end up in Xen (e.g., pvcpuid, int 0x81
hypercalls), and other "hypervisor interactions" will end up in
KVM/qemu (at very least, MMIO accesses from the virtio PCI bus
devices).

Not that it might not be able to be made to work, but I'm not at all
surprised to found out that it doesn't. :-)

If nested virt is your primary goal, you might need to just disable
the virtio device and use normal emulated devices (as you do when
running xen-on-xen).

 -George

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