On Wed, Apr 22, 2020 at 02:14:36PM -0700, Raj, Ashok wrote: > Hi Jason > > > > > > > > > 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, > > > > I'm familiar with the intel naming scheme. > > > > > 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. > > > > Again, if you will have a normal SVA interface, there is no need for a > > VFIO version, just use normal SVA for both. > > > > PCI emulation should try to be in userspace, not the kernel, for > > security. > > Not sure we completely understand your proposal. Mediated devices > are software constructed and they have protected resources like > interrupts and stuff and VFIO already provids abstractions to export > to user space. > > Native SVA is simply passing the process CR3 handle to IOMMU so > IOMMU knows how to walk process page tables, kernel handles things > like page-faults, doing device tlb invalidations and such. > That by itself doesn't translate to what a guest typically does > with a VDEV. There are other control paths that need to be serviced > from the kernel code via VFIO. For speed path operations like > ringing doorbells and such they are directly managed from guest. You don't need vfio to mmap BAR pages to userspace. The unique thing that vfio gives is it provides a way to program the classic non-PASID iommu, which you are not using here. > How do you propose to use the existing SVA api's to also provide > full device emulation as opposed to using an existing infrastructure > that's already in place? You'd provide the 'full device emulation' in userspace (eg qemu), along side all the other device emulation. Device emulation does not belong in the kernel without a very good reason. You get the doorbell BAR page from your own char dev You setup a PASID IOMMU configuration over your own char dev Interrupt delivery is triggering a generic event fd What is VFIO needed for? > Perhaps Alex can ease Jason's concerns? Last we talked Alex also had doubts on what mdev should be used for. It is a feature that seems to lack boundaries, and I'll note that when the discussion came up for VDPA, they eventually choose not to use VFIO. Jason
