Re: [Qemu-devel] [PATCH 0/2] Remove hardcoded xen-platform device initialization (v4)

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

On Thu, Jun 20, 2013 at 05:02:56PM +0200, Paolo Bonzini wrote:
> Il 18/06/2013 16:22, Michael S. Tsirkin ha scritto:
> > On Tue, Jun 18, 2013 at 01:15:57PM +0000, Paul Durrant wrote:
> >>> -----Original Message-----
> >>> From: Laszlo Ersek [mailto:lersek@redhat.com]
> >>> Sent: 18 June 2013 14:14
> >>> To: Michael S. Tsirkin
> >>> Cc: Paul Durrant; qemu-devel@nongnu.org
> >>> Subject: Re: [Qemu-devel] [PATCH 0/2] Remove hardcoded xen-platform
> >>> device initialization (v4)
> >>>
> >>> On 06/18/13 15:01, Michael S. Tsirkin wrote:
> >>>> On Tue, Jun 18, 2013 at 12:57:54PM +0000, Paul Durrant wrote:
> >>>>>> -----Original Message-----
> >>>>>> From: Michael S. Tsirkin [mailto:mst@redhat.com]
> >>>>>> Sent: 18 June 2013 13:52
> >>>>>> To: Laszlo Ersek
> >>>>>> Cc: Paul Durrant; qemu-devel@nongnu.org
> >>>>>> Subject: Re: [Qemu-devel] [PATCH 0/2] Remove hardcoded xen-
> >>> platform
> >>>>>> device initialization (v4)
> >>>>>>
> >>>>>> On Tue, Jun 18, 2013 at 02:37:58PM +0200, Laszlo Ersek wrote:
> >>>>>>> Hi Paul,
> >>>>>>>
> >>>>>>> (xen-devel snipped)
> >>>>>>>
> >>>>>>> On 06/18/13 13:16, Paul Durrant wrote:
> >>>>>>>> Because of concerns over backwards compatibility and a suggestion
> >>> that
> >>>>>>>> xenfv should be retired in favour of using the pc machine type I have
> >>> re-
> >>>>>>>> worked my original patch into 2 patches:
> >>>>>>>>
> >>>>>>>> [PATCH 1/2] Allow use of pc machine type (accel=xen) for Xen HVM
> >>>>>>>> [PATCH 2/2] Move hardcoded initialization of xen-platform device.
> >>>>>>>>
> >>>>>>>> Application of both these patches allows alternative pc machine types
> >>> to
> >>>>>> be
> >>>>>>>> used with the accel=xen option, but preserves the hardcoded
> >>> creation of
> >>>>>>>> the xen-platform device only for machine type xenfv.
> >>>>>>>>
> >>>>>>>> v3:
> >>>>>>>> - Add test for xen_enabled() that went missing in v2
> >>>>>>>>
> >>>>>>>> v4:
> >>>>>>>> - Remove erroneous whitespace hunk
> >>>>>>>> - Replace hw_error() with fprintf()+exit(1)
> >>>>>>>> - Add braces to single-line if
> >>>>>>>
> >>>>>>> can you please offer an opinion in the
> >>>>>>>
> >>>>>>>   [PATCH 1/2] pvpanic: initialization cleanup
> >>>>>>>   http://thread.gmane.org/gmane.comp.emulators.qemu/216940
> >>>>>>>
> >>>>>>> thread?
> >>>>>>>
> >>>>>>> >From where I stand (which is "quite afar" :)) this series of yours seems
> >>>>>>> somewhat related to my doubt there.
> >>>>>>>
> >>>>>>> Thanks!
> >>>>>>> Laszlo
> >>>>>>
> >>>>>> OK will make it skip fwcfg as we did earlier.
> >>>>>> Thanks for the review.
> >>>>>>
> >>>>>
> >>>>> Yes, I think the assert(fw_cfg) would be problematic in the xen case
> >>> where, up until my patch, machine types was necessarily xenfv.
> >>>>>
> >>>>>   Paul
> >>>>
> >>>> Do you guys actually need the pvpanic device?
> >>>> How do you know which port to use without fwcfg?
> >>>
> >>> Xen domains don't know the port and don't use the pvpanic device, but
> >>> qemu starts at least. In other words, the pvpanic device is created, but
> >>> unreachable. Maybe the has_pvpanic logic should depend on (or extended
> >>> with) !xen_enabled().
> >>>
> >>
> >> That seems entirely reasonable to me.
> > 
> > We can just skip creating the device if there's no fw cfg.
> 
> No, in principle Xen domains could use another scheme to find the port
> (xenstore for example).  If Xen domains do not want it, they can just
> add an "if".  Or we could just skip the fw_cfg step.  The device will be
> there but not ACPI-discoverable.
> 
> Paolo

That's a good reason to hide it.  We don't want multiple
mechanisms to discover the same feature, it's a
maintainance headache.

For example, if we do this guests might try to use these ports for something
else by mistake. Or they will start hard-coding the port,
then it will break if user tweaks it from command line.
Simply put, if xen wants to use features exposed through fw cfg,
let's add fw cfg support for xen and use let xen use fw cfg to discover
them.

-- 
MST