Re: [RFC] Independent use of IOMMU groups

[Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>]

Hi Alex,

On Fri, Nov 06, 2015 at 08:35:40AM -0700, Alex Williamson wrote:
> VFIO is really built on iommu groups, so making a vfio group independent
> of iommu groups is a difficult proposition.

I have been thinking about the relation between vfio device groups and
iommu-groups lately, because at least for PCI the iommu-grouping is too
coarse grained. I ran into this with the default-domain approach I am
working on.

Grouping devices together that have different request-ids (multifunction
and acs based grouping) only makes sense when the device is controlled
by an untrusted piece of software, in our case userspace or a KVM guest.
The device drivers in Linux are trusted, and this coarse grained
grouping becomes problematic, because it forces more devices into a
single domain, which can become a bottleneck for DMA-API allocations.

I have been thinking about moving the multi-function and acs grouping
into vfio code, meaning that a vfio-group contains more than one
iommu-group. The problem with this is that iommu-groups are exposed
in sysfs and thus became a userspace ABI.

So the vfio-group code might need changes anyway which could solve the
above problem too, no? I am just not sure yet what the best way is to
solve it.


	Joerg