Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Mar 15, 2017 at 10:36:35AM +0000, Dr. David Alan Gilbert wrote:
> * Markus Armbruster (armbru@redhat.com) wrote:
> > "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
> > 
> > > * Markus Armbruster (armbru@redhat.com) wrote:
> > >> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
> > [...]
> > >> > I'm confused why we need a 'reset to default' - all we need is the ability
> > >> > to change each parameter, and for the new value of that parameter
> > >> > to be an empty string.
> > >> 
> > >> You argue syntax, I'm arguing semantics.
> > >> 
> > >> The command means "set parameter P to value V".  *Except* when V is "",
> > >> it means something else, namely "reset parameter P to its default,
> > >> whatever that may be".
> > >> 
> > >> This is (a) not general, because it won't do for cases where "" may
> > >> occur as value, and (b) ugly.
> > >> 
> > >> Ugliness is the eye of the beholder.  Lack of generality isn't.
> > >
> > > No, I'm questioning why it's defined as 'reset parameter P to its default';
> > > why do we need a way to do that?
> > 
> > Dan's commit message explains:
> > 
> >     Some of the migration parameters are strings, which default to NULL,
> >     eg tls_hostname and tls_creds.
> > 
> >     The mgmt app will set the tls_creds parameter on both source and target
> >     QEMU instances, in order to trigger use of TLS for migration.
> > 
> >     After performing a TLS encrypted migration though, migration might be
> >     used for other reasons - for example, to save the QEMU state to a file.
> >     We need TLS turned off when doing this, but the migrate-set-parameters
> >     QAPI command does not provide any facility to clear/reset parameters
> >     to their default state.
> > 
> >     If you simply ommit the tls_creds parameter in migrate-set-parameters,
> >     then 'has_tls_creds' will be false and so no action will be taken. The
> >     only option that works with migrate-set-parameters is to treat "" on
> >     the wire as equivalent to requesting NULL. Failing that we would have
> >     to create a new 'migrate-reset-parameters' method to explicitly put
> >     a parameter back to its default value.
> 
> OK, I thought the default was empty string.
> IMHO the problem here is we're just being too clever; lets just make
> the default "" rather than NULL, and make that parameter always be
> a string.

>From an internal migration code POV I think NULL is right, but none the
less I'll create a patch showing what using "" internally would look
like so we can compare

Regards,
Daniel
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