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

[Alex Bligh <alex@xxxxxxxxxxx>]

Ian,

> I'm a bit fuzzy on the details but I'm not sure what this has to do with
> the host, the device naming and behaviour on unplug are kernel side
> things, I'd expect that if /dev/sdaX as /boot worked on 3.3 it'll work
> on 4.1 too. (I believe you that it doesn't work, I'm just wondering
> aloud what I'm missing).
>
> Can you give us the specifics of a setup which fails, e.g. a complete
> guest cfg file, the kernel version, command line options, /etc/fstab,
> dmesg etc.
I am not avoiding answering your question (I will get you this) but
what is /meant/ to happen in the following scenario:

* Install on recent kernel (e.g. 2.6.37) running on Xen 3.3
* No fancy boot options, xen_emul_unplug not set
* No XEN_IOPORT_MAGIC implemented, so check_platform_magic()
 returns an error
* Therefore xen_platform_pci_unplug=0
* Therefore blkfront etc. don't init (probe returns
 -ENODEV)
* Therefore OS boots with root=/dev/sda

Now Xen 3.3 is upgraded to Xen 4
* Kernel boots, and XEN_IOPORT_MAGIC now exists
* Therefore unplug occurs, and xen_platform_pci_unplug is non zero
* Therefore blkfront etc. inits, and PV drivers start
* OS still boots with root=/dev/sda

Are the PV devices meant to appear as /dev/sdX rather than /dev/xvdX?
If so, how in the code does this happen? If not, won't the boot fail?

> Hmm, yes I think the special treatment of XEN_IOPORT_MAGIC mismatch on
> the kernel side is what I was missing.
>
> It might make sense to have a guest level config option which disables
> these magic ports, i.e. makes them return 0xffff like they would have
> done in 3.3 (I think 0xffff is what you'll get from an invalid port in
> general).
Actually I don't think this will work. If we do this,
xen_plaftofm_pci_unplug will still be zero (as it's only set on exit
of the function after a successful unplug), and that's enough to
prevent blkfront and xenbus_probe_frontend from doing anything useful,
so will effectively disable PV drivers even where they should be enabled.

> > Out of interest, with a default guest Ubuntu Natty install CD, using the
> > default Xen 4.1 settings, we are seeing the guest (a) unplugging the
> > emulated devices (fine), then (b) failing to find the emulated devices,
> > and (c) the install failing. Is that to be expected?
> Sounds like an Ubuntu bug to me, but I don't follow Ubuntu closely
> enough to know if it is known or not.
We will investigate further. Currently we can't seem to get /any/
distro using PV drivers, and only old ones using emulated drivers.

--
Alex Bligh

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