David M. Brean wrote:
The IBA provides two mechanisms for updating subnet management data:
1) through the verbs - see Modify HCA (section 11.2.1.3) 2) through Subnet management packets (SMPs) - see Subnet Management Class (section 14.2)
The IBA only supports updating the LID via SMPs (#2 above) and an entity using SMPs must have the M_Key. If that entity doesn't have the M_Key, then it can't reliably change the LID.
In addition, the IBA allows an endnode to request, through the verbs
interface provided for the "node reinitialization" (see 14.4.4)
mechanism, that subnet management state, such as the LID, be preserved, when a port transitions through the DOWN state. However, the SM may not honor that request so the endnode must handle that possibility because LID assignment policy is owned by the SM. Furthermore, this mechanism is used on ports that have previously been initialized by the SM (maybe that's why it's called the reinitialization function :)).
Given the mechanisms in the specification, I think that its possible to have IB clients use loopback, even under the endnode power-up scenario, while the port is not in the ACTIVE state and have them continue without disruption when the port is made ACTIVE on the subnet by the SM with use of the reinitialization mechanism. This is a very useful mechanism for various failover situations.
Loopback with reinitialization properly applied....
-David
There is no current IBA mechanism or protocol for an endnode to set just the LID, even if it had the M_Key, and have the SM preserve that value.
-David
Roland Dreier wrote:
I don't see anything in the spec that forbids a CA from having an arbitrary value in PortInfo:LID after initialization but before the SM discovery (please correct me if I missed something). I also don't see anything that forbids an SM implementation from providing a mechanism for preserving the LIDs it finds or administratively assigning LIDs.
Of course none of this is required but I don't see a problem with allowing it.
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