On Fri, Oct 09, 2015 at 12:40:56PM -0600, Alex Williamson wrote: > Recent patches for UIO have been attempting to add MSI/X support, > which unfortunately implies DMA support, which users have been > enabling anyway, but was never intended for UIO. VFIO on the other > hand expects an IOMMU to provide isolation of devices, but provides > a much more complete device interface, which already supports full > MSI/X support. There's really no way to support userspace drivers > with DMA capable devices without an IOMMU to protect the host, but > we can at least think about doing it in a way that properly taints > the kernel and avoids creating new code duplicating existing code, > that does have a supportable use case. > > The diffstat is only so large because I moved vfio.c to vfio_core.c > so I could more easily keep the module named vfio.ko while keeping > the bulk of the no-iommu support in a separate file that can be > optionally compiled. We're really looking at a couple hundred lines > of mostly stub code. The VFIO_NOIOMMU_IOMMU could certainly be > expanded to do page pinning and virt_to_bus() translation, but I > didn't want to complicate anything yet.
I think it's already useful like this, since all current users seem happy enough to just use hugetlbfs to do pinning, and ignore translation. > I've only compiled this and tested loading the module with the new > no-iommu mode enabled, I haven't actually tried to port a DPDK > driver to it, though it ought to be a pretty obvious mix of the > existing UIO and VFIO versions (set the IOMMU, but avoid using it > for mapping, use however bus translations are done w/ UIO). The core > vfio device file is still /dev/vfio/vfio, but all the groups become > /dev/vfio-noiommu/$GROUP. > > It should be obvious, but I always feel obligated to state that this > does not and will not ever enable device assignment to virtual > machines on non-IOMMU capable platforms. In theory, it's kind of possible using paravirtualization. Within guest, you'd make map_page retrieve the io address from the host and return that as dma_addr_t. The only question would be APIs that require more than one contigious page in IO space (e.g. I think alloc coherent is like this?). Not a problem if host is using hugetlbfs, but if not, I guess we could add a hypercall and some Linux API on the host to trigger compaction on the host aggressively. MADV_CONTIGIOUS? > I'm curious what IOMMU folks think of this. This hack is really > only possible because we don't use iommu_ops for regular DMA, so we > can hijack it fairly safely. I believe that's intended to change > though, so this may not be practical long term. Thanks, > > Alex > > --- > > Alex Williamson (2): > vfio: Move vfio.c vfio_core.c > vfio: Include no-iommu mode > > > drivers/vfio/Kconfig | 15 > drivers/vfio/Makefile | 4 > drivers/vfio/vfio.c | 1640 ------------------------------------------ > drivers/vfio/vfio_core.c | 1680 > +++++++++++++++++++++++++++++++++++++++++++ > drivers/vfio/vfio_noiommu.c | 185 +++++ > drivers/vfio/vfio_private.h | 31 + > include/uapi/linux/vfio.h | 2 > 7 files changed, 1917 insertions(+), 1640 deletions(-) > delete mode 100644 drivers/vfio/vfio.c > create mode 100644 drivers/vfio/vfio_core.c > create mode 100644 drivers/vfio/vfio_noiommu.c > create mode 100644 drivers/vfio/vfio_private.h _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
