Re: [Qemu-devel] [RFC PATCH RDMA support v5: 03/12] comprehensive protocol documentation

[Paolo Bonzini <pbonzini@redhat.com>]

Il 15/04/2013 03:10, Michael R. Hines ha scritto:
>>>
>> And when someone writes them one day, we'll have to carry the old code
>> around for interoperability as well. Not pretty.  To avoid that, you
>> need to explicitly say in the documenation that it's experimental and
>> unsupported.
>>
> 
> That's what protocols are for.
> 
> As I've already said, I've incorporated this into the design of the
> protocol
> already.
> 
> The protocol already has a field called "repeat" which allows a user to
> request multiple chunk registrations at the same time.
> 
> If you insist, I can add a capability / command to the protocol called
> "unregister chunk",
> but I'm not volunteering to implement that command as I don't have any data
> showing it to be of any value.

Implementing it on the destination side would be of value because it
would make the implementation interoperable.

A very basic implementation would be "during the bulk phase, unregister
the previous chunk every time you register a chunk".  It would work
great when migrating an idle guest, for example.  It would probably be
faster than TCP (which is now at 4.2 Gbps).

On one hand this should not block merging the patches; on the other
hand, "agreeing to disagree" without having done any test is not very
fruitful.  You can disagree on the priorities (and I agree with you on
this), but what mst is proposing is absolutely reasonable.

Paolo

> That would insulate the protocol against any such future "registration
> cache" design.