Re: [Qemu-devel] [PATCH v4 24/47] Allow savevm handlers to state whether they could go into postcopy

[David Gibson]

On Wed, Nov 19, 2014 at 05:53:54PM +0000, Dr. David Alan Gilbert wrote:
> * David Gibson (address@hidden) wrote:
> > On Fri, Oct 03, 2014 at 06:47:30PM +0100, Dr. David Alan Gilbert (git) 
> > wrote:
> > > From: "Dr. David Alan Gilbert" <address@hidden>
> > > 
> > > Use that to split the qemu_savevm_state_pending counts into postcopiable
> > > and non-postcopiable amounts
> > > 
> > > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > > ---
> > >  arch_init.c                 |  7 +++++++
> > >  include/migration/vmstate.h |  2 +-
> > >  include/sysemu/sysemu.h     |  4 +++-
> > >  migration.c                 |  9 ++++++++-
> > >  savevm.c                    | 23 +++++++++++++++++++----
> > >  5 files changed, 38 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/arch_init.c b/arch_init.c
> > > index 6970733..44072d8 100644
> > > --- a/arch_init.c
> > > +++ b/arch_init.c
> > > @@ -1192,6 +1192,12 @@ static int ram_load(QEMUFile *f, void *opaque, int 
> > > version_id)
> > >      return ret;
> > >  }
> > >  
> > > +/* RAM's always up for postcopying */
> > > +static bool ram_can_postcopy(void *opaque)
> > > +{
> > > +    return true;
> > > +}
> > > +
> > >  static SaveVMHandlers savevm_ram_handlers = {
> > >      .save_live_setup = ram_save_setup,
> > >      .save_live_iterate = ram_save_iterate,
> > > @@ -1199,6 +1205,7 @@ static SaveVMHandlers savevm_ram_handlers = {
> > >      .save_live_pending = ram_save_pending,
> > >      .load_state = ram_load,
> > >      .cancel = ram_migration_cancel,
> > > +    .can_postcopy = ram_can_postcopy,
> > 
> > Is there actually any plausible device for which you'd need a callback
> > here, rather than just having a static bool?
> > 
> > On the other hand, it does seem kind of plausible that there might be
> > situations in which some data from a device must be pre-copied, but
> > more can be post-copied, which would necessitate extending the
> > per-handler callback to return quantities for both.
> 
> It's cheap enough and I couldn't make a strong argument about
> any possible device, so I just used the function.

Ok.  I still wonder if it might be better to instead extend
the save_live_pending callback in order to return both
non-postcopyable and postcopyable quantites.  It allows for the case
of a postcopyable device which has some non-postcopyable data - and
with any postcopyable device other than RAM, it seems likely that
there will need to be some precopied metadata at least.  Plus it
avoids adding another callback.

-- 
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

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