Re: [RFC] /dev/ioasid uAPI proposal

[Jason Gunthorpe]

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