Re: [Qemu-devel] [PATCH v5 20/45] Modify savevm handlers for postcopy

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

* Dr. David Alan Gilbert (dgilbert@redhat.com) wrote:
> * David Gibson (david@gibson.dropbear.id.au) wrote:
> > On Tue, Mar 24, 2015 at 08:04:14PM +0000, Dr. David Alan Gilbert wrote:
> > > * David Gibson (david@gibson.dropbear.id.au) wrote:
> > > > On Fri, Mar 20, 2015 at 12:37:59PM +0000, Dr. David Alan Gilbert wrote:
> > > > > * David Gibson (david@gibson.dropbear.id.au) wrote:
> > > > > > On Fri, Mar 13, 2015 at 10:19:54AM +0000, Dr. David Alan Gilbert wrote:
> > > > > > > * David Gibson (david@gibson.dropbear.id.au) wrote:
> > > > > > > > On Wed, Feb 25, 2015 at 04:51:43PM +0000, Dr. David Alan Gilbert (git) wrote:
> > > > > > > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > > > > > > > 
> > > > > > > > > Modify save_live_pending to return separate postcopiable and
> > > > > > > > > non-postcopiable counts.
> > > > > > > > > 
> > > > > > > > > Add 'can_postcopy' to allow a device to state if it can postcopy
> > > > > > > > 
> > > > > > > > What's the purpose of the can_postcopy callback?  There are no callers
> > > > > > > > in this patch - is it still necessary with the change to
> > > > > > > > save_live_pending?
> > > > > > > 
> > > > > > > The patch 'qemu_savevm_state_complete: Postcopy changes' uses
> > > > > > > it in qemu_savevm_state_postcopy_complete and qemu_savevm_state_complete
> > > > > > > to decide which devices must be completed at that point.
> > > > > > 
> > > > > > Couldn't they check for non-zero postcopiable state from
> > > > > > save_live_pending instead?
> > > > > 
> > > > > That would be a bit weird.
> > > > > 
> > > > > At the moment for each device we call the:
> > > > >        save_live_setup method (from qemu_savevm_state_begin)
> > > > > 
> > > > >    0...multiple times we call:
> > > > >        save_live_pending
> > > > >        save_live_iterate
> > > > > 
> > > > >    and then we always call
> > > > >        save_live_complete
> > > > > 
> > > > > 
> > > > > To my mind we have to call save_live_complete for any device
> > > > > that we've called save_live_setup on (maybe it allocated something
> > > > > in _setup that it clears up in _complete).
> > > > > 
> > > > > save_live_pending could perfectly well return 0 remaining at the end of
> > > > > the migrate for our device, and thus if we used that then we wouldn't
> > > > > call save_live_complete.
> > > > 
> > > > Um.. I don't follow.  I was suggesting that at the precopy->postcopy
> > > > transition point you call save_live_complete for everything that
> > > > reports 0 post-copiable state.
> > > > 
> > > > 
> > > > Then again, a different approach would be to split the
> > > > save_live_complete hook into (possibly NULL) "complete precopy" and
> > > > "complete postcopy" hooks.  The core would ensure that every chunk of
> > > > state has both completion hooks called (unless NULL).  That might also
> > > > address my concerns about the no longer entirely accurate
> > > > save_live_complete function name.
> > > 
> > > OK, that one I prefer.  Are you OK with:
> > >     qemu_savevm_state_complete_precopy
> > >        calls -> save_live_complete_precopy
> > > 
> > >     qemu_savevm_state_complete_postcopy
> > >        calls -> save_live_complete_postcopy
> > > 
> > > ?
> > 
> > Sounds ok to me.  Fwiw, I was thinking that both the complete_precopy
> > and complete_postcopy hooks should always be called.  For a
> > non-postcopy migration, the postcopy hooks would just be called
> > immediately after the precopy hooks.
> 
> OK, I've made the change as described in my last mail; but I haven't called
> the complete_postcopy hook in the precopy case.  If it was as simple as making
> all devices use one or the other then it would work, however there are
> existing (precopy) assumptions about ordering of device state on the wire that
> I want to be careful not to alter; for example RAM must come first is the one
> I know.

Actually, I spoke too soon; testing this found a bad breakage.

the functions in savevm.c add the per-section headers, and then call the _complete
methods on the devices.  Those _complete methods can't elect to do nothing, because
a header has already been planted.

I've ended up with something between the two;  we still have a complete_precopy and
complete_postcopy method on the devices; if the complete_postcopy method exists and
we're in postcopy mode, the complete_precopy method isn't called at all.
A device could decide to do something different in complete_postcopy from complete_precopy
but it must do something to complete the section.
Effectively the presence of the complete_postcopy is now doing what
can_postcopy() used to do.

Dave

> 
> Dave
> 
> > 
> > -- 
> > David Gibson			| I'll have my music baroque, and my code
> > david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> > 				| _way_ _around_!
> > http://www.ozlabs.org/~dgibson
> 
> 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK