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.
