On Tue, Jul 25, 2023 at 04:15:39PM +1000, Benjamin Herrenschmidt wrote: > > Assuming this is for #2, I think VFIO has fallen into a bit of a trap > > by allowing userspace to form the mmap offset. I've seen this happen > > in other subsystems too. It seems like a good idea then you realize > > you need more stuff in the mmap space and become sad. > > > > Typically the way out is to covert the mmap offset into a cookie where > > userspace issues some ioctl and then the ioctl returns an opaque mmap > > offset to use. > > > > eg in the vfio context you'd do some 'prepare region for mmap' ioctl > > where you could specify flags. The kernel would encode the flags in > > the cookie and then mmap would do the right thing. Adding more stuff > > is done by enhancing the prepare ioctl. > > > > Legacy mmap offsets are kept working. > > This indeed what I have in mind. IE. VFIO has legacy regions and add-on > regions though the latter is currently only exploited by some drivers > that create their own add-on regions. My proposal is to add an ioctl to > create them from userspace as "children" of an existing driver-provided > region, allowing to set different attributes for mmap. I wouldn't call it children, you are just getting a different mmap cookie for the same region object. > In the current VFIO the implementation is *entirely* in vfio_pci_core > for PCI and entirely in vfio_platform_common.c for platform, so while > the same ioctls could be imagined to create sub-regions, it would have > to be completely implemented twice unless we do a lot of heavy lifting > to move some of that region stuff into common code. The machinery for managing the mmap cookies should be in common code Jason
