Re: xen_emul_unplug on xen 4.1, HVM guest 2.6.38

[Stefan Bader <stefan.bader@canonical.com>]

On 27.10.2011 15:42, Stefano Stabellini wrote:
> I take we are still talking about PV on HVM guests here.
> 
> On Thu, 27 Oct 2011, Stefan Bader wrote:
>> At least one part is not Ubuntu specific. And that is that the unplug logic
>> decides to unplug emulated devices based on having the pci and the blkfront
>> driver *available* (built-in or module). But later on the blkfront driver
>> ignores all devices that are not *named* in a way to map to the xvd major.
>> Which leaves you without any usable devices when you named your disk hda in
>> the config file and you do not prevent unplugging.
> 
> If you name your disk hda (as you should), blkfront is going to create
> /dev/xvda in your guest.
> It is not ignoring your disk, it just using "xvd" to name the device in
> the guest.
> 
> 
>> Still I would love to see this unplug handling become a bit more obvious. If
>> unplug was successful, then blkfront should not ignore the devices. Or maybe
>> just make the config more what-you-write-is-what-you-get and having hd or sd
>> there only gives you emulated devices and xvd gives you pv devices.
> 
> Yes, if the unplug is unsuccessful blkfront should not ignore the
> device: it is going to create a /dev/xvd* for you.

The problem I saw in my test was that in blkfront_probe the following case was
hit when the device name was hda in the cfg:

                if (xen_platform_pci_unplug & XEN_UNPLUG_UNNECESSARY) {
                        int major;

                        if (!VDEV_IS_EXTENDED(vdevice))
                                major = BLKIF_MAJOR(vdevice);
                        else
                                major = XENVBD_MAJOR;

                        if (major != XENVBD_MAJOR) {
                                printk(KERN_INFO
                                                "%s: HVM does not support vbd %d
as xen block device\n",
                                                __FUNCTION__, vdevice);
                                return -ENODEV;
                        }
                }

So major is not XENVBD_MAJOR and the device is ignored.

-Stefan