On Wed, Apr 22, 2020 at 2:24 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > On Tue, Apr 21, 2020 at 4:55 PM Jason Gunthorpe <jgg@xxxxxxxxxxxx> 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.
