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

[Tian, Kevin]

> From: Jason Gunthorpe
> Sent: Wednesday, April 22, 2020 7:55 AM
> 
> 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.

Well, this series is for virtualizing idxd device to VMs, instead of supporting
SVA for bare metal processes. idxd implements a hardware-assisted 
mediated device technique called Intel Scalable I/O Virtualization, which
allows each Assignable Device Interface (ADI, e.g. a work queue) tagged 
with an unique PASID to ensure fine-grained DMA isolation when those 
ADIs are assigned to different VMs. For this purpose idxd utilizes the VFIO 
mdev framework and IOMMU aux-domain extension. Bare metal SVA will
be enabled for idxd later by using the general SVA code that you mentioned.
Both paths will co-exist in the end so there is no such case of disabling
DMA/iommu config.

Thanks
Kevin

> 
> 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.
> 
> Jason