Re: [Qemu-devel] [SeaBIOS] [PATCH] don't expose pvpanic device in the UI

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 06, 2013 at 12:20:27PM +0300, Gleb Natapov wrote:
> On Tue, Aug 06, 2013 at 10:45:01AM +0200, Andreas Färber wrote:
> > Am 06.08.2013 10:36, schrieb Gleb Natapov:
> > > On Tue, Aug 06, 2013 at 11:33:10AM +0300, Michael S. Tsirkin wrote:
> > >> On Tue, Aug 06, 2013 at 10:21:52AM +0300, Gleb Natapov wrote:
> > >>>> If you see a mouse in a room, how likely is it that there's
> > >>>> a single mouse there?
> > >>>>
> > >>>> This is a PV technology which to me looks like it was
> > >>>> rushed through and not only set on by default, but
> > >>>> without a way to disable it - apparently on the assumption
> > >>>> there's 0 chance it can cause any damage. Now that
> > >>>> we do know the chance it's not there, why not go back
> > >>>> to the standard interface, and why not give
> > >>>> users a chance to enable/disable it?
> > >>> You should be able to disable it with: -device pvpanic,ioport=0
> > >>
> > >> Doesn't work for me.
> > > Bug that should be fixed. With this command line _STA should return
> > > zero.
> > > 
> > >> Besides, both -device pvpanic and use of ioport=0 to disable it
> > >> are completely undocumented.
> > >>
> > > Not the only undocumented thing in QEMU command line :)
> > [snip]
> > 
> > I disagree: -device adds a device, not removes one. It will still be
> > present.
> > 
> I assume you are answering to the quote about ioport=0, not
> documentation here. ioport=0 does not removes the device, it disables
> it. The claim was it cannot be disabled, it can (assuming no bugs), but
> it should not be.
> 
> > I am neutral as to whether qemu-system-x86_64 should have it enabled by
> > default or not. But if we want to suppress it, then -nodefaults should
> > disable it. Since libvirt uses that though, it would mean libvirt would
> > need to add it back, whether via user's XML domain config or by libvirt
> > itself based on some version/etc. heuristics.
> > 
> There is no clear definition of what -nodefaults should or should not
> do.
> Should it disable PV ACPI hotplug device?
> Should it create a machine
> with no mmio/io regions registered at all?

We can't change the meaning for existing devices.
So we need to keep everything that it historically
included.
But it's a good question whether e.g. ACPI hotplug
for q35 should be a separate device, and disabled
with -nodefaults.

> 
> --
> 			Gleb.

pvpanic presents itself as a separate device, this is different from
PV ACPI hotplug which is part of PIIX.