Re: [Xen-devel] Xen 4.x / Linux 3.x (dom0 and HVM domU) and NIC handling

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2011-12-05 at 14:55 +0000, Stefan Bader wrote:
> On 02.12.2011 18:16, Alex Bligh wrote:
> > 
> > 
> > --On 2 December 2011 16:40:48 +0000 Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
> > wrote:
> > 
> >>> AFAIK changing xen_platform_pci=0|1 will switch rather more than just
> >>> the NIC. It will switch your disk too, instantly causing your previously
> >>> happily booting OS to fail to boot as the root device name changes.
> >>
> >> We recommend you use "root=LABEL=foo" rather than "root=/dev/blah" for
> >> this reason. Fortunately most distros use that scheme by default these
> >> days.
> > 
> > Yes; and /etc/fstab. UUID= works too.
> > 
> > FWIW my experience is that various built-for-cloud type distros don't use
> > that scheme, mainly because they use grub1 which IIRC does not support
> > this, and building images in a non-root environment that have grub1
> > in is rather easier than grub2. So, for instance, all the vm-builder
> > stuff in debian/ubuntu used grub1 and did not work this way.
> > 
> > However, my point was that xen_platform_pci does not only change
> > whether your net driver is emulated or PVHVM, but also whether your
> > disk, and indeed everything else is emulated or PVHVM.
> > 
> I can understand a policy of using the pv devices whenever it is possible. The
> change of the device name is there but as pointed out most distro 
> installations
> try to avoid those anyway since there is a similar problem with usb keys or
> drives potentially moving around. Same for network devices that get mapped 
> based
> on mac address.
> Not sure how real the need for a mixed setup is.

Basically non-existent IMHO. The option to not use pv devices is more
for debugging or recovering in the case that you can't boot from the pv
device for some reason (e.g. because you aren't using LABEL/UUID= ;-)).

> If, then I can see that it gets
> a bit weird. While you can use xen_emul_unplug with other keywords to prevent
> unplugging disks or nics. But that would not remove the related pv devices.

It does (at least in upstream Linux) prevent the PV driver from
initialising, which isn't quite the same but serves the same end
purpose.

Ian.

>  And
> I am not sure whether this would be a desired behavior or actually be feasible
> in a clean layered way. Personally I would think nobody could want both
> interfaces at the same time but I would not assume I know all of the use 
> cases.
> 
> -Stefan



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