On Tue, Jun 08, 2021 at 12:37:04PM +1000, David Gibson wrote: > > The PPC/SPAPR support allows KVM to associate a vfio group to an IOMMU > > page table so that it can handle iotlb programming from pre-registered > > memory without trapping out to userspace. > > To clarify that's a guest side logical vIOMMU page table which is > partially managed by KVM. This is an optimization - things can work > without it, but it means guest iomap/unmap becomes a hot path because > each map/unmap hypercall has to go > guest -> KVM -> qemu -> VFIO > > So there are multiple context transitions. Isn't this overhead true of many of the vIOMMUs? Can the fast path be generalized? Jason
