[libvirt PATCH] qemu_migration: Delete vDPA check

[Eugenio Perez Martin]

On Tue, Jul 19, 2022 at 6:43 PM Laine Stump <laine at laine.org> 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 at laine.org> 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 at redhat.com> 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.
> >
>