On Wed, Jun 23, 2021 at 07:59:21AM +0000, Tian, Kevin wrote: > > From: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> > > Sent: Thursday, June 17, 2021 11:48 AM > > > > On Tue, Jun 08, 2021 at 10:17:56AM -0300, Jason Gunthorpe wrote: > > > 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? > > > > Yes, but historically it bit much harder on POWER for a couple of reasons: > > > > 1) POWER guests *always* have a vIOMMU - the platform has no concept > > of passthrough mode. We therefore had a vIOMMU implementation some > > time before the AMD or Intel IOMMUs were implemented as vIOMMUs in > > qemu. > > > > 2) At the time we were implementing this the supported IOVA window for > > the paravirtualized IOMMU was pretty small (1G, I think) making > > vIOMMU maps and unmaps a pretty common operation. > > > > > Can the fast path be > > > generalized? > > > > Not really. This is a paravirtualized guest IOMMU, so it's a platform > > specific group of hypercalls that's being interpreted by KVM and > > passed through to the IOMMU side using essentially the same backend > > that that the userspace implementation would eventually get to after a > > bunch more context switches. > > > > Can virtio-iommu work on PPC? iirc Jean has a plan to implement > a vhost-iommu which is supposed to implement the similar in-kernel > acceleration... I don't know - I'd have to research virtio-iommu a bunch to determine that. Even if we can, the platform IOMMU would still be there (it's a platform requirement), so we couldn't completely ignore it. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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