Re: [libvirt PATCH] qemu_migration: Delete vDPA check

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(a)laine.org&gt; 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(a)redhat.com&gt; 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/4003b7047058a17465083178d6c0...

> > 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).

> 3) a patch that adds a call to that function to > qemuMigrationSrcIsAllowed(). >

> 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). >

> I had thought there was a bugzilla somewhere that was tracking this for > libvirt, but I can't find it. >