linux-kernel - RE: [patch 21/32] NTB/msi: Convert to msi_on_each

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Date:   Thu, 09 Dec 2021 10:03:00 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Marc Zygnier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Dey, Megha" <megha.dey@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Mason <jdmason@...zu.us>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>,
        "linux-ntb@...glegroups.com" <linux-ntb@...glegroups.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "x86@...nel.org" <x86@...nel.org>, Joerg Roedel <jroedel@...e.de>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: RE: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

Kevin,

On Thu, Dec 09 2021 at 06:26, Kevin Tian wrote:
>> From: Jason Gunthorpe <jgg@...dia.com>
>> 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