Re: 4.7 qemu regression: HVM guests fail to boot from xvda

[George Dunlap <george.dunlap@citrix.com>]

On 03/06/16 12:45, Ian Jackson wrote:
> George Dunlap writes ("Re: [Xen-devel] 4.7 qemu regression: HVM guests fail to boot from xvda"):
>> On 03/06/16 12:20, Olaf Hering wrote:
>>> I think the regression is: 'vdev=xvda' does not result in a disk
>>> connected to the emulated controller. Should we change the way hdtype=
>>> is handled internally? If hdtype= is not given it remains unset and with
>>> vdev=xvd* no disk-on-emulated-controller gets added. If hdtype= is set
>>> then vdev=xvd* will result in an disk-on-emulated-controller, which
>>> fixes the regression. If vdev=hd* and hdtype= was not set, hdtype will
>>> be silently set to ide.
>>
>> I'd be OK with this.  But is the "hdtype unset" also available at the
>> libxl level?
> 
> There are two problems with this `hdtype' approach.
> 
> Firstly, it is global.  That is, it applies to all disks of the
> particular guest.  But then maybe we don't care about that because
> this anomalous major-number-stealing behaviour is probably per-guest
> rather than per-disk.
> 
> Secondly, the proposal above involves changing both the semantics of
> existing `hdtype' parameter values, and the default hdtype value.  The
> resulting situation would be that even specifying vdev=hda wouldn't
> get you an emulated device, by default, unless you specified `hdtype'
> too.  I don't think that is right.

I don't quite understand this.

First of all, if I make a disk with "vdev=xvda,hdtype=ide", what
happens?  I presume that the 'hdtype' field is effectively ignored?

Secondly, why would the "vdev=hda" behavior change under Olaf's suggestion?

I think what he's proposing (and again this is from a xl.cfg level, not
a libxl level) is this:

* "vdev=xvda": You get only a PV device.  Under both XenoLinux and
upstream Linux your PV device is named 'xvda'.  (No change from existing
semantics.)

* "vdev=hda": You get an emulated IDE "backed" by a PV device.  Under
XenoLinux your PV device is named 'hda'.  Under upstream Linux your PV
device is named 'xvda' (No change from existing semantics.)

* "vdev=xvda,hdtype=ide": You get an emulated IDE backed by a PV device.
 Under both XenoLinux and upstream Linux your PV device is named 'xvda'.
 (This is the only change.)

At a libxl level, the exact same functionality is possible to enable, right?

 -George

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