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Only iommufd can implement an importer, > > the kernel module loader will enforce this. > > > > Allow anything to implement an exporter as there are use cases in > > DPDK/SPDK to connect GPU memory into VFIO/iommufd and it is hard to abuse > > the API as an exporter. > > > > The physical address list exporter returns a physical address list in a > > simple kvalloc'd array of struct phys_vec. > > As far as I can see that is still a pretty big NO-GO. > > We have seen so many problems with direct physical address access by > the importer that I clear don't want to repeat that performance. You've said, this is why I used the EXPORT_SYMBOL_FOR_MODULES() - iommufd does not have any problems to use this correctly. Why is that not good enough? As I understand it your objection isn't that there is a technical issue with iommufd's implementation it is that some other driver could import phys, do it wrong and make a mess. I think EXPORT_SYMBOL_FOR_MODULES() fully addresses this, no? The only past problems you've raised were related to improperly using VMAs, that isn't happening here. > My main question is why does IOMMUFD need the physical address in > the first place? CPU iommu hw only works in physical address. > If that is really strictly necessary then I strongly suggest to not > touch drivers/dma-buf in any way, but only do this is private > interface between iommufd and KVM. This isn't between iommufd and kvm. If it was just that we'd probably be able to keep going with the private path like VFIO/iommufd already has. The two paths we are interested in: 1) VFIO to KVM Allow KVM to import a FD instead of using a VMA like iommufd now can. 2) GPU and RDMA drivers to IOMMUFD Support SPDK and DPDK type userspace drivers to work with GPU memory This is becoming a popular topic Jason