On Thu, Feb 25, 2021 at 03:21:13PM -0700, Alex Williamson wrote: > This is where it gets tricky. The vm_pgoff we get from > file_operations.mmap is already essentially describing an offset from > the base of a specific resource. We could convert that from an absolute > offset to a pfn offset, but it's only the bus driver code (ex. > vfio-pci) that knows how to get the base, assuming there is a single > base per region (we can't assume enough bits per region to store > absolute pfn). Also note that you're suggesting that all vfio mmaps > would need to standardize on the vfio-pci implementation of region > layouts. Not that most drivers haven't copied vfio-pci, but we've > specifically avoided exposing it as a fixed uAPI such that we could have > the flexibility for a bus driver to implement regions offsets however > they need. Okay, well the bus driver owns the address space and the bus driver is in control of the vm_pgoff. If it doesn't want to zap then it doesn't need to do anything vfio-pci can consistently use the index encoding and be fine > So I'm not really sure what this looks like. Within vfio-pci we could > keep the index bits in place to allow unmmap_mapping_range() to > selectively zap matching vm_pgoffs but expanding that to a vfio > standard such that the IOMMU backend can also extract a pfn looks very > limiting, or ugly. Thanks, Lets add a op to convert a vma into a PFN range. The map code will pass the vma to the op and get back a pfn (or failure). pci will switch the vm_pgoff to an index, find the bar base and compute the pfn. It is starting to look more and more like dma buf though Jason
