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

[Aaron Lindsay]

On Oct 16 09:21, Dr. David Alan Gilbert wrote:
> * Richard Henderson (address@hidden) 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 <address@hidden>
> > > ---
> > >  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?

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?

-Aaron