Xen users - Dec 2013 - XenProject.org Q&A: Running xen to create PV hosts under KVM with virtio

The following question was posed by Simon Easter on the
XenProject.org Q&A area.  Please reply to me so I can post your
answers to the site or, if you feel so inclined, answer yourself on
the website:

I have a need to run very low weight PV debian guests under a kvm
hypervised guest .(i.e. split a vm using pv - the reasons are varied
but essentially each host has to have it''s own networking/routing
layer)
I can boot these only when the underlying block device in kvm guest is
switched to ide from virtio.(on test system)

My question is: is there any way I can get xen to boot with virtio as
I don''t have the control to switch the block device supplied to IDE
(it''s not permitted)
Can the xen system be recompiled or can I compile a module that works
and doesn''t result in error message of

udevd timout: killing ‘/sbin/modprobe -v virtio………

Any suggestions I was going to look at jails but am unsure the
networking is self contained in each jail so xen pv was the next
choice.

Simon

If I understand him correctly he wants to use virtio as the "host"
(from
Xen's PoV) disk device driver (i.e. the KVM guest's I/O path). As
distinct from wanting to use virtio as the I/O path for the Xen PV
guests.

My experience with the ARM Foundation model (which also exposes a virtio
"host" disk) is that the virtio drivers do not work under Xen because
they do not correctly use the kernel's DMA APIs (this is needed because
under Xen physical addrs to not map 1:1 to DMA addresses). By "do not
correctly use" I actually mean "do not use at all".

IMHO this is a bug in virtio and it would be lovely to see it fixed, but
AFAIK noone is working on that since it is a bit of a niche use case. If
he wants to have a stab at it he'll probably want to join the xen-devel
and qemu-devel lists.

On ARM we have the option to make DMA addr=physical addr which can be
used to work around this issue. I don't think there is any similar
option for x86 PV dom0.

Ian.

On Wed, 2013-12-11 at 09:48 -0500, Russ Pavlicek wrote:
>  The following question was posed by Simon Easter on the
> XenProject.org Q&A area.  Please reply to me so I can post your
> answers to the site or, if you feel so inclined, answer yourself on
> the website:
> 
> I have a need to run very low weight PV debian guests under a kvm
> hypervised guest .(i.e. split a vm using pv - the reasons are varied
> but essentially each host has to have it's own networking/routing
> layer)
> I can boot these only when the underlying block device in kvm guest is
> switched to ide from virtio.(on test system)
> 
> My question is: is there any way I can get xen to boot with virtio as
> I don't have the control to switch the block device supplied to IDE
> (it's not permitted)
> Can the xen system be recompiled or can I compile a module that works
> and doesn't result in error message of
> 
> udevd timout: killing ‘/sbin/modprobe -v virtio………
> 
> Any suggestions I was going to look at jails but am unsure the
> networking is self contained in each jail so xen pv was the next
> choice.
> 
> Simon
> 
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xen.org
> http://lists.xen.org/xen-users


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