Re: Questions about hvm domU default devices with upstream qemu

[George Dunlap <dunlapg@umich.edu>]

On Thu, Sep 5, 2013 at 12:23 PM, Stefano Stabellini
<stefano.stabellini@eu.citrix.com> wrote:
> On Wed, 4 Sep 2013, Fabio Fantoni wrote:
>> Il 04/09/2013 15:17, Stefano Stabellini ha scritto:
>> > On Wed, 4 Sep 2013, Fabio Fantoni wrote:
>> > > Il 04/07/2013 15:51, Fabio Fantoni ha scritto:
>> > > > Last year I posted a question about default devices of upstream qemu
>> > > > that
>> > > > differ from qemu traditional, like empty floppy and cdrom in particular.
>> > > > About empty floppy now is disabled as workaround.
>> > > > About empty cdrom Stabellini tells that is useful, so I tried to use it
>> > > > but
>> > > > cd-insert was failing,and I must define other empty cdrom on xl
>> > > > configuration file, for example with: ',raw,hdb,ro,cdrom'.
>> > > > It seem that there isn't default devices usable without define them in
>> > > > xen,
>> > > > and I think that probably need to revise the definition of the devices
>> > > > to
>> > > > avoid empty and unusable ones and to do everything as best as possible
>> > > > (also
>> > > > with possibly reviewing qemu side).
>> > > > Another good thing is also consider pv domU case and possible future
>> > > > support
>> > > > of spice and/or other useful features.
>> > > > Thanks for any reply.
>> > > I seen no replies about this, I think is important improve upstream qemu
>> > > support and remove the traditional ASA upstream will be equivalent or
>> > > superior
>> > > in all features.
>> > >
>> > > I also think is good idea add q35 support on xen.
>> > > Seem that hvm domUs have performance limits, in particular on windows
>> > > domUs
>> > > (also with pv drivers) caused probably by old hardware emulation by qemu
>> > > and/or xen support limited on some instruction sets.
>> > > I don't know how to add essential q35 support on hvmloader to do some
>> > > tests
>> > > and have effective data about improvements.
>> > > Anyone on this?
>> > I agree that this is important and thanks for raising the issue.
>> > It would be nice if somebody stepped up to do the q35 work.
>>
>> Thanks for reply, can you reply also on questions about default devices with
>> upstream qemu please?
>
> I think it might be a good idea to keep the same set of default devices
> between qemu-traditional and qemu-xen.
>
> We always had an empty cdrom and I don't see why we should remove it. If
> cd-insert doesn't work, it's a bug and should be fixed.

It sounds similar to the floppy disk thing: if you don't specifically
tell qemu that you do *not* want a cdrom, it will give you one; but
since xl doesn't know anything about it, you can't actually use it.

It seems like there needs to be a "--no-default-devices" option for
qemu that will say, "No really, only put in what I ask you to put in."

>
> Supporting spice in PV guests would be hard, because spice needs QXL
> that is a PCI device. PV guests don't have a PCI bus. Of course if
> somebody did the work to make QXL and spice work with PV guests I would
> be happy to accept it.

If I recall correctly, isn't part of the problem that the PCI access
essentially requires synchronous communication via MMIO accesses?
We'd basically have to add a device model and MMIO handling to PV
guests.  Possible but far from simple.  (Possibly easier for PVH
domains once they come out.)

 -George