[libvirt PATCH] qemu_migration: Delete vDPA check

[Eugenio Perez Martin]

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.

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?

Thanks!