Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams

[Juan Quintela <quintela@redhat.com>]

Markus Armbruster <armbru@redhat.com> wrote:
> Juan Quintela <quintela@redhat.com> writes:
>
>> Eric Blake <eblake@redhat.com> wrote:


>>> Or is the proposal that we are also going to simplify the QMP 'migrate'
>>> command to get rid of crufty parameters?
>>
>> I didn't read it that way, but I would not oppose O:-)
>>
>> Later, Juan.
>
> I'm not too familiar with this stuff, so please correct my
> misunderstandings.
>
> "Normal" migration configuration is global state, i.e. it applies to all
> future migrations.
>
> Except the "migrate" command's flags apply to just the migration kicked
> off by that command.
>
> QMP command "migrate" has two flags "blk" (HMP: -b) and "inc" (HMP: -i).
> !blk && inc makes no sense and is silently treated like !blk && !inc.
>
> There's a third flag "detach" (HMP: -d), but it does nothing in QMP.

As qmp command is asynchronous, you can think that -d is *always* on in
QMP O:-)

> You'd like to deprecate these flags in favour of "normal" configuration.
> However, we need to maintain QMP backward compatibility at least for a
> while.  HMP backward compatibility is nice to have, but not required.
>
> First step is to design the new interface you want.  Second step is to
> figure out backward compatibility.
>
> The new interface adds a block migration tri-state (off,
> non-incremental, incremental) to global state, default off.  Whether
> it's done as two bools or an enum of three values doesn't matter here.

Tristates will complicate it.  I still think that:

- capability: block_migration
- parameter: block_shared

Makes more sense, no?

If block_migration is not enabled, we ignore the shared parameter.  We
already do that for other parameters.

> If the new interface isn't used, the old one still needs to work.  If it
> is used, the old one either has to do "the right thing", or fail
> cleanly.
>
> We approximate "new interface isn't used" by "block migration is off in
> global state".  When it is off, the migration command needs to honor its
> two flags for compatibility.  It must leave block migration off in
> global state.  Yes, this will complicate the implementation until we
> actually remove the deprecated flags.  Par for the backward compatility
> course.
>
> When block migration isn't off in global state, we can either
>
> * let the flags take precedence over the global state (one
>   interpretation of "do the right thing"), or
>
> * reject flags that conflict with global state (another interpretation),
>   or
>
> * reject *all* flags (fail cleanly).
>
> The last one looks perfectly servicable to me.

Yeap,  I think that makes sense.  If you use capabilities, parameters,
old interface don't work at all.

We still have a problem that is what happens if the user does:

migrate -b <foo>
migrate_cancel (or error)
migrate <bar> (without -b)

With current patches, it will still use -b.  Fixing it requires still
anding more code.  But I think that this use case is so weird what we
should not even care about it.

Later, Juan.