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, Alex
