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

[Dr. David Alan Gilbert]

* David Gibson (address@hidden) wrote:
> On Fri, Mar 20, 2015 at 12:37:59PM +0000, Dr. David Alan Gilbert wrote:
> > * David Gibson (address@hidden) wrote:
> > > On Fri, Mar 13, 2015 at 10:19:54AM +0000, Dr. David Alan Gilbert wrote:
> > > > * David Gibson (address@hidden) wrote:
> > > > > On Wed, Feb 25, 2015 at 04:51:43PM +0000, Dr. David Alan Gilbert 
> > > > > (git) wrote:
> > > > > > From: "Dr. David Alan Gilbert" <address@hidden>
> > > > > > 
> > > > > > 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

?

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 / address@hidden / Manchester, UK