On Thu, Jun 22, 2017 at 11:27:09AM -0600, Alex Williamson wrote: > On Thu, 22 Jun 2017 22:42:19 +0530 > Nitin Saxena <nitin....@gmail.com> wrote: > > > Thanks Alex. > > > > >> Without an iommu in the VM, you'd be limited to no-iommu support for VM > > >> userspace, > > So are you trying to say VFIO NO-IOMMU should work inside VM. Does > > that mean VFIO NO-IOMMU in VM and VFIO IOMMU in host for same device > > is a legitimate configuration? I did tried this configuration and the > > application (in VM) seems to get container_fd, group_fd, device_fd > > successfully but after VFIO_DEVICE_RESET ioctl the PCI link breaks > > from VM as well as from host. This could be specific to PCI endpoint > > device which I can dig. > > > > I will be happy if VFIO NO-IOMMU in VM and IOMMU in host for same > > device is legitimate configuration. > > Using no-iommu in the guest should work in that configuration, however > there's no isolation from the user to the rest of VM memory, so the VM > kernel will be tainted. Host memory does have iommu isolation. Device > reset from VM userspace sounds like another bug to investigate. Thanks, > > Alex
Besides what Alex has mentioned, there is a wiki page for the usage. The command line will be slightly different on QEMU side comparing to without vIOMMU: http://wiki.qemu.org/Features/VT-d#With_Assigned_Devices One more thing to mention is that, when vfio-pci devices in the guest are used with emulated VT-d, huge performance degradation will be expected for dynamic allocations at least for now. While for mostly static allocations (like DPDK) the performance should be merely the same as no-IOMMU mode. It's just a hint on performance, and I believe for your own case it should mostly depend on how the application is managing DMA map/unmaps. Thanks, -- Peter Xu
