Messages in this thread | | | Date | Tue, 31 May 2022 16:01:04 +0100 | Subject | Re: [PATCH 01/12] iommu/vt-d: Use iommu_get_domain_for_dev() in debugfs | From | Robin Murphy <> |
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On 2022-05-31 15:53, Jason Gunthorpe wrote: > 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..
Eww, no, we can't kill performance by forcing serialisation on the entire API just for one silly driver-internal debugfs corner :(
Robin.
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