RE: [RFC] /dev/ioasid uAPI proposal

Tian, Kevin
2021-06-23T07:59:30Z

> From: David Gibson <david@gibson.dropbear.id.au>
> 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...

Thanks
Kevin