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

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Message-ID: <20211202144514.GQ4670@nvidia.com>
Date:   Thu, 2 Dec 2021 10:45:14 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Kevin Tian <kevin.tian@...el.com>,
        Megha Dey <megha.dey@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-pci@...r.kernel.org,
        Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>, linux-ntb@...glegroups.com,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>, x86@...nel.org,
        Joerg Roedel <jroedel@...e.de>,
        iommu@...ts.linux-foundation.org
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

On Thu, Dec 02, 2021 at 03:23:38PM +0100, Greg Kroah-Hartman wrote:
> On Thu, Dec 02, 2021 at 09:55:02AM -0400, Jason Gunthorpe wrote:
> > Further, there is no reason why IMS should be reserved exclusively for
> > VFIO! Why shouldn't the cdev be able to use IMS vectors too? It is
> > just a feature of the PCI device like MSI. If the queue has a PASID it
> > can use IDXD's IMS.
> 
> No, sorry, but a cdev is not for anything resembling any real resource
> at all.

My point is that when the user asks the driver to allocate a queue
through a cdev ioctl it should be able to get the queue attached to an
IMS, today it can only get a queue attached to a MSI.

> It is ONLY for the /dev/NODE interface that controls the character
> device api to userspace.  The struct device involved in it is ONLY for
> that, nothing else.  Any attempt to add things to it will be gleefully
> rejected.

I agree with you!
 
> > If we really need a 2nd struct device to turn on IMS then, I'd suggest
> > picking the cdev, as it keeps IMS and its allocator inside the IDXD
> > PCIe driver and not in the VFIO world.
> 
> No!  Again, a cdev is to control the lifespan/lifecycle of the /dev/NODE
> only.  Anything other than that is not ok to do at all.

Said the same thing in a prior email - which is why I think the only
logical choice here is to make IMS work on the pci_device

FWIW I feel the same way about the VFIO mdev - its *ONLY* purpose is
to control the lifecycle and we are close to stripping away all the
other abuses using it for other things.

Jason