Re: [Qemu-devel] [PATCH v6 03/14] migration: Add post_save function to VMStateDescription

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

* Aaron Lindsay (aclindsa@gmail.com) wrote:
> On Oct 16 09:21, Dr. David Alan Gilbert wrote:
> > * Richard Henderson (richard.henderson@linaro.org) wrote:
> > > On 10/10/18 1:37 PM, Aaron Lindsay wrote:
> > > > In some cases it may be helpful to modify state before saving it for
> > > > migration, and then modify the state back after it has been saved. The
> > > > existing pre_save function provides half of this functionality. This
> > > > patch adds a post_save function to provide the second half.
> > > > 
> > > > Signed-off-by: Aaron Lindsay <aclindsa@gmail.com>
> > > > ---
> > > >  docs/devel/migration.rst    |  9 +++++++--
> > > >  include/migration/vmstate.h |  1 +
> > > >  migration/vmstate.c         | 10 +++++++++-
> > > >  3 files changed, 17 insertions(+), 3 deletions(-)
> > > 
> > > Hmm, maybe.  I believe the common practice is for pre_save to copy state into a
> > > separate member on the side, so that conversion back isn't necessary.
> > > 
> > > Ccing in the migration maintainers for a second opinion.
> > 
> > It is common to copy stuff into a separate member; however we do
> > occasionally think that post_save would be a useful addition; so I think
> > we should take it (if nothing else it actually makes stuff symmetric!).
> > 
> > Please make it return 'int' in the same way that pre_save/pre_load
> > does, so that it can fail and stop the migration.
> 
> This patch calls post_save *even if the save operation fails*. My
> reasoning was that I didn't want a failed migration to leave a
> still-running original QEMU instance in an invalid state. Was this
> misguided?

That's fine - my only issue is that I want post_save to be able to fail
itself even if pre_save failed.

> If it was not, which error do you prefer to be returned from
> vmstate_save_state_v() in the case that both the save operation itself
> and the post_save call returned errors?

The return value from the save operation.
I did wonder about suggesting that you pass the return value from the
save operation as a parameter to post_save.

Dave

> -Aaron
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK