On Tue, Jul 19, 2022 at 6:43 PM Laine Stump <laine@xxxxxxxxx> wrote: > > On 7/19/22 12:01 PM, Laine Stump wrote: > > On 7/19/22 11:09 AM, Eugenio Perez Martin wrote: > >> On Tue, Jul 19, 2022 at 4:02 PM Laine Stump <laine@xxxxxxxxx> wrote: > >>> > >>> On 7/18/22 11:15 AM, Jiri Denemark wrote: > >>>> On Mon, Jul 18, 2022 at 10:40:56 +0200, Eugenio Perez Martin wrote: > >>>>> On Mon, Jul 18, 2022 at 10:25 AM Jiri Denemark > >>>>> <jdenemar@xxxxxxxxxx> wrote: > >>>>>> Which in ideal case would mean only a QMP command (such as > >>>>>> hotplugging a non-migratable device) is the only way to add migration > >>>>>> blocker. If this is true, than we're safe as libvirt does not > >>>>>> allow such > >>>>>> commands between qemuMigrationSrcIsAllowed and migration start. > >>>>>> > >>>>> > >>>>> Ok, that rules out a few bad use cases. I can do a fast lookup to > >>>>> check if blockers can be added without the knowledge of libvirt. > >>>>> > >>>>>> That said, is there a reason for not implementing the correct > >>>>>> solution > >>>>>> right away as a separate patch? > >>>>>> > >>>>> > >>>>> I was not sure if libvirt already had another way to check, for > >>>>> example, if the vhost device didn't have VHOST_F_LOG_ALL feature. > >>>> > >>>> I'm not aware of such check, but even if it exists, checking for > >>>> migration blockers looks like the right way of doing things anyway. > >>> > >>> Actually that's been on my todo list for a long time - for any qemu that > >>> supports the QMP command that checks for migratability, we should be > >>> calling this command rather than checking against our own internal list > >>> (which is really just an "informed guess") of what can't be migrated. > >>> This way we'll always get the right answer (or at least what QEMU > >>> believes to be the right answer :-)). Fixing it this way will also mean > >>> that migration of VFIO devices will just "magically" start working once > >>> a migration-supporting driver is written for the device, and the correct > >>> vfio driver is bound to the device (this latter item is also on my > >>> list). > >>> > >>> So if you're up for making the patch to call the QMP command, I'd be > >>> happy to review it! > >>> > >> > >> Thanks! Actually I'd need some guidance first, I'm not very used to > >> libvirt code. > >> > >> As I understand I should create a function in qemu_agent.h/c, a getter > >> similar to qemuAgentGetFSInfo. How can I get a qemuAgent from > >> qemuMigrationSrcIsAllowed? I only have a virQEMUDriver there. > > > > qemu_agent.c is only for functions that require calling to the QEMU > > guest agent, which is a process running inside the guest. You just need > > to run a simple QMP command. There are some good examples of this in > > qemu_monitor_json.c > > > >> > >> For now it should be enough to delete vdpa hardcoded negation, and > >> then other parts of libvirt can delete other hardcoded checks, isn't > >> it? > > > > There's just a single function that checks for migratability > > (qemuMigrationSrcIsAllowed()). In theory *everything* in that function > > should be deprecated by just calling qemu to ask. In practice there may > > be / probably are things that qemu doesn't count as "can't migrate" that > > really should be counted that way. Certainly the VDPA and hostdev checks > > should be removable immediately though (although of course this should > > still be checked before pushing!) > > > > > > What I would do is this: > > > > 1) a patch that adds code to the qemu_capabilities to set a flag if the > > desired field in the "query-migrate" QMP command would be filled in by > > this qemu binary. > > Just to permanently document live discussions from IRC: > > jjongsma pointed us to a patch he wrote a year ago (but never pushed > upstream) that implements (1): > > https://gitlab.com/jjongsma/libvirt/-/commit/4003b7047058a17465083178d6c0a0f62c900c74 > Thanks! > > > > 2) a patch that adds a function to qemu_monitor_json.c to call that > > command and return migratable/not. > > Thinking about this more, I guess a function that returns the full text > of "blocked-reason" would be more useful (that way it could be easily > logged). > I'm actually returned all the array in the form of `char ***` > > 3) a patch that adds a call to that function to > > qemuMigrationSrcIsAllowed(). > > What I cannot find an example of is how to get the qemuMonitor from the virQEMUDriver that qemuMigrationSrcIsAllowed has as the parameter. > > 4) additional patches that remove specific hardcoded checks *only if the > > new field in query-migrate is available (as indicated by the new > > capabilities flag) and returned a definite yes/no (otherwise the checks > > still need to be done, to account for older qemu binaries that don't > > have the qmp command). > > Yes, I agree. Thanks! > > I had thought there was a bugzilla somewhere that was tracking this for > > libvirt, but I can't find it. > > >
