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

[Markus Armbruster <armbru@redhat.com>]

"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.