Re: [PATCH v4 15/32] vfio: introduce KVM-owned IOMMU type

[Matthew Rosato]

On 3/15/22 1:25 PM, Jason Gunthorpe wrote:
> On Tue, Mar 15, 2022 at 12:29:02PM -0400, Matthew Rosato wrote:
> > On 3/15/22 10:38 AM, Jason Gunthorpe wrote:
> > > On Tue, Mar 15, 2022 at 09:49:01AM -0400, Matthew Rosato wrote:
> > >
> > > > The rationale for splitting steps 1 and 2 are that VFIO_SET_IOMMU doesn't
> > > > have a mechanism for specifying more than the type as an arg, no?  Otherwise
> > > > yes, you could specify a kvm fd at this point and it would have some other
> > > > advantages (e.g. skip notifier).  But we still can't use the IOMMU for
> > > > mapping until step 3.
> > >
> > > Stuff like this is why I'd be much happier if this could join our
> > > iommfd project so we can have clean modeling of the multiple iommu_domains.
> >
> > I'd certainly be willing to collaborate so feel free to loop me in on the
> > discussions;
>
> Sure, I have you on my list. I've been waiting for Eric to get a bit
> further along on his ARM work so you have something appropriate to
> look at.
>
> In the mean time you can certainly work out the driver details as
> you've been doing here and hacking through VFIO. The iommu_domain
> logic is the big work item in this series, not the integration with
> the uAPI.

A subset of this series (enabling some s390x firmware-assist facilities) 
is not tied to the iommu and would still provide value while continuing 
to use vfio_iommu_type1 for all mapping -- so I think I'll look into a 
next version that shrinks down to that subset (+ re-visit the setup API).

Separate from that, I will continue looking at implementing the nested 
iommu_domain logic for s390, and continue to hack through VFIO for now. 
I'll use an RFC series when I have something more to look at, likely 
starting with the fully-pinned guest as you suggest; ultimately I'm 
interested in both scenarios (pinned kvm guest & dynamic pinning during 
shadow)

Thanks,
Matt