Messages in this thread | | | Date | Mon, 14 Feb 2022 14:10:19 +0000 | Subject | Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain() | From | Robin Murphy <> |
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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.
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