Re: [RFC] /dev/ioasid uAPI proposal

[David Gibson]

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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