On Tue, Oct 10, 2017 at 03:01:10PM -0300, Eduardo Habkost wrote: > 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.
It certainly feels like a device. It has state (that needs to be migrated), it has a host/guest interface. > > > > 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. Certainly not a fw cfg flag. Can be a machine flag I guess but then we'd have to open-code each such device. And don't forget auto - this is what Daniel asks for. I don't necessarily see this device as so special and think a generic interface to control what goes into the machine would be better (e.g. look how you use hacky -global to control fw cfg options, it only works if there's a single one), but if everyone thinks otherwise and agrees we should have it in there by default, and a property to disable, fine. Can be a patch on top though. > -- > Eduardo
