On Tue, Oct 10, 2017 at 04:06:28PM +0100, Daniel P. Berrange wrote: > 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 > > <berra...@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" <berra...@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.
It's still possible to add a -machine option that can enable/disable automatic creation of the device. But I also don't see why it needs to be implemented using -device if it's not really a device. A boolean machine or fw_cfg property is good enough for that. > > 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. Both "-global fw_cfg.vmcoreinfo=on|off" and "-machine vmcoreinfo=on|off" sound good enough to me. -- Eduardo
