Messages in this thread | | | Date | Tue, 31 May 2022 11:53:01 -0300 | From | Jason Gunthorpe <> | Subject | Re: [PATCH 01/12] iommu/vt-d: Use iommu_get_domain_for_dev() in debugfs |
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On Tue, May 31, 2022 at 10:11:18PM +0800, Baolu Lu wrote: > On 2022/5/31 21:10, Jason Gunthorpe wrote: > > On Tue, May 31, 2022 at 11:02:06AM +0800, Baolu Lu wrote: > > > > > For case 2, it is a bit weird. I tried to add a rwsem lock to make the > > > iommu_unmap() and dumping tables in debugfs exclusive. This does not > > > work because debugfs may depend on the DMA of the devices to work. It > > > seems that what we can do is to allow this race, but when we traverse > > > the page table in debugfs, we will check the validity of the physical > > > address retrieved from the page table entry. Then, the worst case is to > > > print some useless information. > > > > Sounds horrible, don't you have locking around the IOPTEs of some > > kind? How does updating them work reliably? > > There's no locking around updating the IOPTEs. The basic assumption is > that at any time, there's only a single thread manipulating the mappings > of the range specified in iommu_map/unmap() APIs. Therefore, the race > only exists when multiple ranges share some high-level IOPTEs. The IOMMU > driver updates those IOPTEs using the compare-and-exchange atomic > operation.
Oh? Did I miss where that was documented as part of the iommu API?
Daniel posted patches for VFIO to multi-thread iommu_domin mapping.
iommufd goes out of its way to avoid this kind of serialization so that userspace can parallel map IOVA.
I think if this is the requirement then the iommu API needs to provide a lock around the domain for the driver..
Jason
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