Re: [Qemu-devel] [PATCH v6 2/7] hw/misc: add vmcoreinfo device

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Oct 10, 2017 at 05:00:18PM +0200, Marc-André Lureau wrote:
> Hi
> 
> On Tue, Oct 10, 2017 at 10:31 AM, Daniel P. Berrange
> <berrange@redhat.com> wrote:
> > On Tue, Oct 10, 2017 at 12:44:26AM +0300, Michael S. Tsirkin wrote:
> >> On Mon, Oct 09, 2017 at 02:02:18PM +0100, Daniel P. Berrange wrote:
> >> > On Mon, Oct 09, 2017 at 02:43:44PM +0200, Igor Mammedov wrote:
> >> > > On Mon, 9 Oct 2017 12:03:36 +0100
> >> > > "Daniel P. Berrange" <berrange@redhat.com> wrote:
> >> > >
> >> > > > On Mon, Sep 11, 2017 at 06:59:24PM +0200, Marc-André Lureau wrote:
> >> > > > > See docs/specs/vmcoreinfo.txt for details.
> >> > > > >
> >> > > > > "etc/vmcoreinfo" fw_cfg entry is added when using "-device vmcoreinfo".
> >> > > >
> >> > > > I'm wondering if you considered just adding the entry to fw_cfg by
> >> > > > default, without requiring any -device arg ? Unless I'm misunderstanding,
> >> > > > this doesn't feel like a device to me - its just a well known bucket
> >> > > > in fw_cfg IIUC ?  Obviously its existance would need to be tied to
> >> > > > the latest machine type for ABI reasons though. The benefit of this
> >> > > > is that it would "just work" without us having to plumb it through to
> >> > > > all the downstream applications that use QEMU for mgmt guest (OpenStack,
> >> > > > oVirt, GNOME Boxes, virt-manager, and countless other mgmt apps).
> >> > > it follows model set by pvpanic device, it's easier to manage from migration
> >> > > POV, one could use it even for old machine types with new qemu (just by adding
> >> > > device, it makes instance not backwards migratable to old qemu but should work
> >> > > for forward migration) and if user doesn't need it, device could be just omitted
> >> > > from CLI.
> >> >
> >> > Sure but it means that in effect no one will have this functionality enabled
> >> > for several years. pvpanic has been around a long time and I rarely see it
> >> > present in configured guests :-(
> >> >
> >> >
> >> > Regards,
> >> > Daniel
> >>
> >> libvirt runs with -nodefaults, right? I'd argue pretty strongly -nodefaults
> >> shouldn't add optional devices anyway.
> >
> > This isn't really adding a device though is it - it is just a well known
> > location in fw_cfg to receive data.
> 
> Enabling the device on some configurations by default can be done as a
> follow-up patch. Can we get this series reviewed & merged?

The problem with the -device approach + turning it on by default is that there
is no way to turn it off again if you don't want it. eg there's way to undo
an implicit '-device foo' except via -nodefaults, but since libvirt uses that
already it would negate the effect of enabling it by default unconditionally.

Your previous approach of "-global fw_cfg.vmcoreinfo=on" is nicer in this
respect, as you can trivially turn it on/off, overriding the default state
in both directions.

Regards,
Daniel
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