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

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Date:   Fri, 10 Dec 2021 07:37:41 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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()

> From: Thomas Gleixner <tglx@...utronix.de>
> Sent: Thursday, December 9, 2021 11:58 PM
> 
> On Thu, Dec 09 2021 at 12:17, Kevin Tian wrote:
> >> From: Thomas Gleixner <tglx@...utronix.de>
> >> 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,.....)
> >
> > Not exactly IMS-ENTRY. MSI-ENTRY also works.
> 
> Sure.
> 
> >>
> >> 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.
> >
> > While it's one way of looking at this model do we want to actually
> > create some objects to represent this 'virtual queue' concept? that
> > implies each ENQCMD must be moderated to create such short-lifespan
> > objects and I'm not sure the benefit of doing so.
> 
> You don't have to create anything. The PASID/ENTRY pair represents that
> 'virtual queue', right?

Yes

> 
> > If not then from driver p.o.v it's still one queue resource and driver
> > needs to manage its association with multiple interrupt entries and
> > PASIDs when it's connected to multiple clients.
> 
> That's correct, but there is nothing problematic with it. It's like
> allocating multiple interrupts for any other hardware device or
> subdevice, right?

No question on this. Just want to point out such usage example 
since Jason mentioned it. 😊

> 
> What's probably more interresting is how the PASID/interrupt/RID
> relations are managed.
> 

yes, that's something we need further think of.

Thanks
Kevin