Re: xen_emul_unplug on xen 4.1, HVM guest 2.6.38

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

On 27.10.2011 16:08, Stefano Stabellini wrote:
> On Thu, 27 Oct 2011, Stefan Bader wrote:
>> 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.
>>
> 
> Why are you passing xen_emul_unplug=unnecessary?
> The idea is that if you pass that option you have to use the emulated
> path to access your disk unless you esplicitely add an xvd disk to your
> config.
> 
> In the normal case, if you don't specify xen_emul_unplug, you are going
> to get xvda for your hda disk.
> Then if you use LABEL or UUID in your root= argument, you don't need to
> change any configuration to make it work.

Because of the above I have to specify the xen_emul_unplug to get *any* device.
If I do not specify anything, then (because pci and pv driver are available) the
emulated disk gets unplugged. Ok. But then the probe does not give me any xvd
because the major does not match.

The only form I could make this work was: use xvda in the config file.