* Markus Armbruster (arm...@redhat.com) wrote: > Cc: David for insurance against me spewing nonsense about migration. > > John Snow <js...@redhat.com> writes: > > > On 6/25/20 12:45 AM, Markus Armbruster wrote: > >> John Snow <js...@redhat.com> writes: > >> > >>> On 6/22/20 5:42 AM, Markus Armbruster wrote: > >>>> There are three ways to configure backends: > >>>> > >>>> * -nic, -serial, -drive, ... (onboard devices) > >>>> > >>>> * Set the property with -device, or, if you feel masochistic, with > >>>> -set device (pluggable devices) > >>>> > >>>> * Set the property with -global (both) > >>>> > >>>> The trouble is -global is terrible. > >>>> > >>>> It gets applied in object_new(), which can't fail. We treat failure > >>>> to apply -global as fatal error, except when hot-plugging, where we > >>>> treat it as warning *boggle*. I'm not addressing that today. > >>>> > >>>> Some code falls apart when you use both -global and the other way. > >>>> > >>>> To make life more interesting, we gave -drive two roles: with > >>>> interface type other than none, it's for configuring onboard devices, > >>>> and with interface type none, it's for defining backends for use with > >>>> -device and such. Since we neglect to require interface type none for > >>>> the latter, you can use one -drive in both roles. This confuses the > >>>> code about as much as you, dear reader, probably are by now. > >>>> > >>>> Because this still isn't interesting enough, there's yet another way > >>>> to configure backends, just for floppies: set the floppy controller's > >>>> property. Goes back to the time when floppy wasn't a separate device, > >>>> and involves some Bad Magic. Now -global can interact with itself! > >>>> > >>>> Digging through all this took me an embarrassing amount of time. > >>>> Hair, too. > >>>> > >>>> My patches reject some the silliest uses outright, and deprecate some > >>>> not so silly ones that have replacements. > >>>> > >>>> Apply on top of my "[PATCH v2 00/58] qdev: Rework how we plug into the > >>>> parent bus". > >>>> > >>> > >>> Oof. Thank you for your work in fixing our darkest corners. I sincerely > >>> appreciate it. > >>> > >>> The qdev tree ordering problems don't cause any issues for migration, do > >>> they? > >> > >> This series should only change device configuration, not device state or > >> its encoding in the migration stream. > >> > >> I'm not sure what you mean by "qdev tree ordering problems". Ist it > >> commit e8c9e65816 'qom: Make "info qom-tree" show children sorted'? > >> > >>> (I see you already sent a PR, so whatever!) > >> > >> A question that might avoid a later migration debugging session is > >> *never* "whatever"! > >> > > > > I thought I had read that one of these patches changes the order in > > which devices get instantiated, which I thought might change their QOM > > paths. Which I thought *might* have some ramifications for migration, > > but wasn't sure. > > Device instantiation order changes should not break migration.
They shouldn't; although I only narrowly stopped a new device from making a mistake that would have made it dependent. Of course you do have to explicitly state PCI/USB slot IDs otherwise the allocation of those is order dependent. > The order in which devices appear in the migration stream should not > matter. Order in the stream is a separate issue; we have ways to enforce that; for example you want the interrupt controller to arrive before a device that will raise an interrupt. Dave Dave > > If it's just showing the same path outputs *sorted*, then there's no > > problem. > > > > Likely misread. > > > > --js -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK