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

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Olaf Hering writes ("Re: [Xen-devel] 4.7 qemu regression: HVM guests fail to 
boot from xvda"):
> To achieve this the default hdtype= should become "UNSET", and a vdev=hd
> should set it to IDE if it was "UNSET". That means there could be yet
> another state "PVONLY".

I'm afraid I think this way lies madness.  You are suggesting that the
default for hdtype= should depend in a complicated way on the set of
disk vdevs.  (It also makes hotplug very confusing.)

> > 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*'.
> 
> This would be an backward compatible approach, at least domU.cfg will
> work with older toolstack. libvirt needs to know about this.

This is a strange use of the phrase "backward compatible".  What you
mean is that the necessary domU.cfg changes are backward compatible.

> > What should happen if these options are enabled for PV guests - should
> > they be silently ignored, or should they be rejected ?
> 
> IMO no. Do we have such rejects already for PV or HVM only options?

Maybe.

> It has to be noted that libvirt does not seem to know about the hdtype=
> knob, which was introduced in xen-4.6.

Anyway, to conclude: it seems that you don't like any of my other
options.  I don't like your suggestion.  But you seem happy with my
option (1), above.

Personally I prefer George's suggestion:
  What about a per-disk property, emulate={default,always,only}

Ian.

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