RE: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

[Thomas Gleixner]

Kevin,

On Thu, Dec 09 2021 at 06:26, Kevin Tian wrote:
>> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
>> I don't know of any use case for more than one interrupt on a queue,
>> and if it did come up I'd probably approach it by making the queue
>> handle above also specify the 'queue relative HW index'
>
> We have such use case with IDXD.
>
> Basically the IDXD queue allows software to put an interrupt handle
> (the index of MSI-X or IMS entry) in the submitted descriptor. Upon
> completion of the descriptor the hardware finds the specified entry 
> and then generate interrupt to notify software.
>
> Conceptually descriptors submitted to a same queue can use different
> handles, implying one queue can be associated to multiple interrupts.

I think you are looking at that from the internal implementation details
of IDXD. But you can just model it in an IDXD implementation agnostic
way:

     ENQCMD(PASID, IMS-ENTRY,.....)

implies an on demand allocation of a virtual queue, which is deallocated
when the command completes. The PASID and IMS-ENTRY act as the 'queue'
identifier.

The implementation detail of IDXD that it executes these computations on
an internal shared workqueue does not change that.

Such a workqueue can only execute one enqueued command at a time, which
means that during the execution of a particular command that IDXD
internal workqueue represents the 'virtual queue' which is identified by
the unique PASID/IMS-ENTRY pair.

No?

Thanks,

        tglx