Re: [Xen-users] Help get Xen 4.1 running

["Fajar A. Nugraha" <list@xxxxxxxxx>]

On Thu, Nov 10, 2011 at 10:20 PM, Bud Bundy <budric@xxxxxxxxx> wrote:
> Thanks for your suggestions.
> Adding xen_platform_pci=0 made the network card AND the hard drive
> available.
> You said
>>> If using xen_platform_pci works, it means you're using
>>> qemu-emulated devices
> Can you or someone explain further what this option does?  I want HVM and
> emulated devices.  I plan to install Windows later and this was supposed to
> be a quick test for running HVM machines.  By why isn't builder='hvm' and
> kernel=... and device_model='/usr/lib/xen-default/bin/qemu-dm' enough to
> specify that?  What is it doing when I don't specify xen_platform_pci=0?

It's something related to the fact that if BOTH pv and emulated
devices available (which is the case in old-version linux pv drivers),
users can accidently mount both thus corrupting the data. Or it could
simply make it harder to use the pv device (e.g. when using LVM, it'd
get confused seeing two identical PVs). So in newer versions of Linux,
if pv drivers are available then emulated devices get unplugged.

The problem with that is in some distro xen_platform_pci is compiled
as module, and not included by default in initrd. That means the
kernel remembers that it had xen pv drivers support, thus unplugging
the emulated devices. However because the module is not included in
initrd, the frontend drivers (xen-blkfront, etc) won't load, thus no
devices are available.

xen_platform_pci (in domU config file) controls the visibility of a
device needed (in domU side) for pv drivers to work. If set to 0, pv
drivers won't work, and the emulated devices won't get unplugged.

You should remove the setting (or set it to 1) if:
- you use windows + pv drivers (e.g. GPLPV)
- you use linux, and you compiled xen_platform_pci as builtin, or have
xen_platform_pci module loaded

-- 
Fajar

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