Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()

[Robin Murphy]

On 2022-02-14 12:45, Jason Gunthorpe wrote:
> On Mon, Feb 14, 2022 at 12:09:36PM +0000, Robin Murphy wrote:
> > On 2022-01-06 02:20, Lu Baolu wrote:
> > > Expose an interface to replace the domain of an iommu group for frameworks
> > > like vfio which claims the ownership of the whole iommu group.
> >
> > But if the underlying point is the new expectation that
> > iommu_{attach,detach}_device() operate on the device's whole group where
> > relevant, why should we invent some special mechanism for VFIO to be
> > needlessly inconsistent?
> >
> > I said before that it's trivial for VFIO to resolve a suitable device if it
> > needs to; by now I've actually written the patch ;)
> >
> > https://gitlab.arm.com/linux-arm/linux-rm/-/commit/9f37d8c17c9b606abc96e1f1001c0b97c8b93ed5
>
> Er, how does locking work there? What keeps busdev from being
> concurrently unplugged?

Same thing that prevents the bus pointer from suddenly becoming invalid 
in the current code, I guess :)

But yes, holding a group reference alone can't prevent the group itself 
from changing, and the finer points of locking still need working out - 
there's a reason you got a link to a WIP branch in my tree rather than a 
proper patch in your inbox (TBH at the moment that one represents about 
a 5:1 ratio of time spent on the reasoning behind the commit message vs. 
the implementation itself).

> How can iommu_group_get() be safely called on
> this pointer?

VFIO hardly needs to retrieve the iommu_group from a device which it 
derived from the iommu_group it holds in the first place. What matters 
is being able to call *other* device-based IOMMU API interfaces in the 
long term. And once a robust solution for that is in place, it should 
inevitably work for a device-based attach interface too.

> All of the above only works normally inside a probe/remove context
> where the driver core is blocking concurrent unplug and descruction.
>
> I think I said this last time you brought it up that lifetime was the
> challenge with this idea.

Indeed, but it's a challenge that needs tackling, because the bus-based 
interfaces need to go away. So either we figure it out now and let this 
attach interface rework benefit immediately, or I spend three times as 
long solving it on my own and end up deleting 
iommu_group_replace_domain() in about 6 months' time anyway.

Thanks,
Robin.