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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Wed, 22 Apr 2020 14:24:11 -0700
Message-ID: <CAPcyv4gMYz1wCYjfnujyGXP0jGehpb+dEYV7hJoAAsDsj9+afQ@mail.gmail.com>
Subject: Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS
 support for the idxd driver.
To:     Jason Gunthorpe <jgg@mellanox.com>
Cc:     Dave Jiang <dave.jiang@intel.com>, Vinod Koul <vkoul@kernel.org>,
        Megha Dey <megha.dey@linux.intel.com>, maz@kernel.org,
        Bjorn Helgaas <bhelgaas@google.com>,
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        Greg KH <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Alex Williamson <alex.williamson@redhat.com>,
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On Tue, Apr 21, 2020 at 4:55 PM Jason Gunthorpe <jgg@mellanox.com> wrote:
>
> On Tue, Apr 21, 2020 at 04:33:46PM -0700, Dave Jiang wrote:
> > The actual code is independent of the stage 2 driver code submission that adds
> > support for SVM, ENQCMD(S), PASID, and shared workqueues. This code series will
> > support dedicated workqueue on a guest with no vIOMMU.
> >
> > A new device type "mdev" is introduced for the idxd driver. This allows the wq
> > to be dedicated to the usage of a VFIO mediated device (mdev). Once the work
> > queue (wq) is enabled, an uuid generated by the user can be added to the wq
> > through the uuid sysfs attribute for the wq.  After the association, a mdev can
> > be created using this UUID. The mdev driver code will associate the uuid and
> > setup the mdev on the driver side. When the create operation is successful, the
> > uuid can be passed to qemu. When the guest boots up, it should discover a DSA
> > device when doing PCI discovery.
>
> I'm feeling really skeptical that adding all this PCI config space and
> MMIO BAR emulation to the kernel just to cram this into a VFIO
> interface is a good idea, that kind of stuff is much safer in
> userspace.
>
> Particularly since vfio is not really needed once a driver is using
> the PASID stuff. We already have general code for drivers to use to
> attach a PASID to a mm_struct - and using vfio while disabling all the
> DMA/iommu config really seems like an abuse.
>
> A /dev/idxd char dev that mmaps a bar page and links it to a PASID
> seems a lot simpler and saner kernel wise.
>
> > The mdev utilizes Interrupt Message Store or IMS[3] instead of MSIX for
> > interrupts for the guest. This preserves MSIX for host usages and also allows a
> > significantly larger number of interrupt vectors for guest usage.
>
> I never did get a reply to my earlier remarks on the IMS patches.
>
> The concept of a device specific addr/data table format for MSI is not
> Intel specific. This should be general code. We have a device that can
> use this kind of kernel capability today.

This has been my concern reviewing the implementation. IMS needs more
than one in-tree user to validate degrees of freedom in the api. I had
been missing a second "in-tree user" to validate the scope of the
flexibility that was needed.