On Wed, 20 Nov 2019 12:07:15 -0700 Alex Williamson <alex.williamson@xxxxxxxxxx> wrote: > On Wed, 20 Nov 2019 10:35:03 -0800 > Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx> wrote: > > > On Tue, 19 Nov 2019 15:56:20 -0800 > > "Alex Williamson" <alex.williamson@xxxxxxxxxx> wrote: > > > > > On Mon, 11 Nov 2019 16:45:07 +0800 > > > lantianyu1986@xxxxxxxxx wrote: > > > > > > > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx> > > > > > > > > This patch is to add VFIO VMBUS driver support in order to expose > > > > VMBUS devices to user space drivers(Reference Hyper-V UIO driver). > > > > DPDK now has netvsc PMD driver support and it may get VMBUS resources > > > > via VFIO interface with new driver support. > > > > > > > > So far, Hyper-V doesn't provide virtual IOMMU support and so this > > > > driver needs to be used with VFIO noiommu mode. > > > > > > Let's be clear here, vfio no-iommu mode taints the kernel and was a > > > compromise that we can re-use vfio-pci in its entirety, so it had a > > > high code reuse value for minimal code and maintenance investment. It > > > was certainly not intended to provoke new drivers that rely on this mode > > > of operation. In fact, no-iommu should be discouraged as it provides > > > absolutely no isolation. I'd therefore ask, why should this be in the > > > kernel versus any other unsupportable out of tree driver? It appears > > > almost entirely self contained. Thanks, > > > > > > Alex > > > > The current VMBUS access from userspace is from uio_hv_generic > > there is (and will not be) any out of tree driver for this. > > I'm talking about the driver proposed here. It can only be used in a > mode that taints the kernel that its running on, so why would we sign > up to support 400 lines of code that has no safe way to use it? > > > The new driver from Tianyu is to make VMBUS behave like PCI. > > This simplifies the code for DPDK and other usermode device drivers > > because it can use the same API's for VMBus as is done for PCI. > > But this doesn't re-use the vfio-pci API at all, it explicitly defines > a new vfio-vmbus API over the vfio interfaces. So a user mode driver > might be able to reuse some vfio support, but I don't see how this has > anything to do with PCI. > > > Unfortunately, since Hyper-V does not support virtual IOMMU yet, > > the only usage modle is with no-iommu taint. > > Which is what makes it unsupportable and prompts the question why it > should be included in the mainline kernel as it introduces a > maintenance burden and normalizes a usage model that's unsafe. Thanks, Many existing userspace drivers are unsafe: - out of tree DPDK igb_uio is unsafe. - VFIO with noiommu is unsafe. - hv_uio_generic is unsafe. This new driver is not any better or worse. This sounds like a complete repeat of the discussion that occurred before introducing VFIO noiommu mode. Shouldn't vmbus vfio taint the kernel in the same way as vfio noiommu does?
