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

[Juan Quintela <quintela@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> * 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.

The one from save.  In general, this one shouldn't be called, and if it
gets an error, we are really in big trouble, no?  By big treauble I mean
that we can basically only stop the guest?

Later, Juan.

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