Re: [Xen-devel] 4.7 qemu regression: HVM guests fail to boot from xvda

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

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.

The possibilities I see are:

(1) New boolean per-guest parameter for this behaviour, meaning
   `provide emulated devices for all xvd* as if they were hd*'.

(2) New `hdtype=ideforpv' which has the same effect as `hdtype=ide'
   plus the semantics in (1) above.  (I'm open to better naming
   suggestions.)

(3) New disk property parameter `hvm-emulate' in the Deprecated
    section of xl-disk-configuration.txt.

Open questions:

Do we also need `... as if they were sd*' or `provide ide emulated
devices where vdev=sd* is specified?'

If we have `hdtype=ideinclpv' do we also need `hdtype=ahciinclpv' ?

What should happen if these options are enabled for PV guests - should
they be silently ignored, or should they be rejected ?

Ian.

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