Re: [Xen-devel] xen_emul_unplug on xen 4.1, HVM guest 2.6.38

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Wed, Oct 26, 2011 at 03:56:44PM +0100, Ian Campbell wrote:
> On Wed, 2011-10-26 at 15:47 +0100, Alex Bligh wrote:
> > Pasi, Conrad,
> > 
> > --On 26 October 2011 17:12:33 +0300 Pasi Kärkkäinen <pasik@xxxxxx> wrote:
> > 
> > > On Wed, Oct 26, 2011 at 09:49:42AM -0400, Konrad Rzeszutek Wilk wrote:
> > >> On Wed, Oct 26, 2011 at 02:17:25PM +0100, Alex Bligh wrote:
> > >> > I am running Xen 4 and want users to be able to install arbitrary HVM
> > >> > images. We want both PV and emulated drivers to appear. The reason
> > >> > for this is for compatibility with images that were prepared for
> > >> > Xen 3.3, which assume emulated devices exist at boot time (particularly
> > >> > for running the boot loader).
> > >> >
> > >> > If I do not specify on the boot line xen_emul_unplug=never, then
> > >> > under some guest OS's (e.g. Ubuntu Natty (2.6.38)), the domU kernel
> > >> > unplugs the non-PV devices, which is undesirable in my application.
> > >> >
> > >> > I cannot arbitrarily change the kernel command line because it is
> > >> > in effect set by the user's image.
> > >> >
> > >> > Therefore what I need to do is prevent Xen4 offering the facility
> > >> > to unplug the devices in the first place. Setting "(unpluggable 0)"
> > >> > in the config for the emulated NICs and disks does not appear
> > >> > to prevent them getting unplugged; what may be happening is that
> > >> > domU tries to unplug them (but fails) but still doesn't use them.
> > >> > Is there a way to signal from dom0 that the kernel really shouldn't
> > >> > unplug this stuff.
> > >>
> > >> xen_pci_platform=0 in your guest config should do it.
> > >>
> > >
> > > It's actually called:
> > > xen_platform_pci=0
> > 
> > Thanks. Just to confirm, that will still allow HVM PV drivers to
> > operate (as well)?
> 
> No, this will disable PV drivers.
> 
> The decision to unplug is a kernel side decision and in PVHVM Linux
> kernels is not currently possible to have both types of devices by
> default due to the risk of dataloss if the guest is not correctly
> configured (i.e. the kernel can't tell if it is mounting the same
> filesystem via two paths). The xen_emul_unplug option is the current way
> you can override this once you have confirmed that your guest
> configuration is not dangerous. I'm afraid this necessarily involves
> guest config and guest admin interaction.
> 
> In principal we might be able to extend the unplug protocol (which would
> involve patches to qemu, the kernel(s) and the toolstack) to allow
> devices to be marked as being not necessary to unplug. Someone would
> have to send patches though and it would be opening up a way for people
> to lose data so we'd need to be careful.
> 
> I'm sure that the unplug protocol is documented somewhere in the source
> tree but I can't for the life of me find it :-(
> 

"disable qemu PCI devices in HVM domains":
http://xenbits.xen.org/gitweb/?p=qemu-xen-unstable.git;a=commit;h=e7911109f4321e9ba0cc56a253b653600aa46bea

"disable qemu PCI devices in HVM domains - protocol document":
http://xenbits.xen.org/gitweb/?p=qemu-xen-unstable.git;a=commit;h=bc5c93ede039525765a7aa98e1da3386efa1465f


-- Pasi



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