Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS support for the idxd driver.

Dan Williams
2020-04-23T19:18:05Z

On Wed, Apr 22, 2020 at 2:24 PM Dan Williams <dan.j.williams@intel.com> wrote:
>
> 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.

Hey Jason,

Per Megha's follow-up can you send the details about that other device
and help clear a path for a device-specific MSI addr/data table
format. Ever since HMM I've been sensitive, perhaps overly-sensitive,
to claims about future upstream users. The fact that you have an
additional use case is golden for pushing this into a common area and
validating the scope of the proposed API.