Fwd: Re: [openib-general] static LID computation withTS_HOST_DRIVER

[Michael Krause]

At 03:40 PM 9/30/2004, 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.

This is a reasonable approach where the loopback LID being used is updated 
upon the port being initialized (akin to solving this in the CI but still 
allowing CM to work with a known LID.  It avoids any complexity in the SM 
having to preserve LID that may not be optimal or potentially unique within 
the subnet.

Not sure this might work but it seems to me that APM mech could be used to 
configure a new configured LID and then transfer the connection to the 
configured.  May take a bit of work in CM as APM is nominally set up during 
these exchanges.

>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.

Agreed.

Mike


>-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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