Re: [Qemu-devel] [PATCH 2/4] migration/savevm: use migration_is_blocked to validate

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Wei Yang (richardw.yang@linux.intel.com) wrote:
> On Thu, Apr 25, 2019 at 05:55:15PM -0300, Daniel Henrique Barboza wrote:
> >
> >
> >On 4/23/19 9:46 PM, Wei Yang wrote:
> >> migration_is_blocked() is used in migrate_prepare() and
> >> save_snapshot(), this is more proper to use this instead of
> >> qemu_savevm_state_blocked() in qemu_loadvm_state().
> >
> >
> >migration_is_blocked() does an additional verification:
> >
> >"if (migration_blockers)"
> >
> >comparing to what was previously done in qemu_loadvm_state.
> >
> >I've checked what migration_blockers does and it is a GList used
> >for callers to block the migration process. This is used via
> >'migration_add_blocker', from migration.c.
> >
> >'migration_add_blocker' is called all over the place, most notably
> >in  _realize() functions  and _open() functions from block.
> >
> >Thus, I am not sure if this change will impact the use of
> >qemu_loadvm_state() from load_snapshot() (i.e. can load_snapshot
> >be called with migration_blockers?). It's better to someone
> >with a better understanding of this code to comment on that.
> >
> 
> Well, when you look into the source side of migration:
> 
> qmp_migrate
>   migrate_prepare
>     migration_is_blocked
> 
> This means if migration_is_blocked fails, the source will not start migration.
> And it is the same as save_snapshot.
> 
> From my understanding, when we load a vm, it should check the same
> requirement.

I've been thinking about this, and I think I agree with Daniel on this.
The 'migration_blockers' list tells you that something about the
*current* state of a device means that it can't be migrated - e.g.
a 9pfs with a mounted filesystem can't be migrated.

If we're about to reload the state from a snapshot, then the saved
snapshot's state must have been migratable, so that's OK.

(Whether all the device code is actually OK about being reset in
that state is a different question; but I think it should be).

Dave

> -- 
> Wei Yang
> Help you, Help me
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK