On Mon, Aug 08, 2022 at 02:55:28PM -0300, Jason Gunthorpe wrote: > On Mon, Aug 08, 2022 at 08:10:22AM +0200, Greg Kroah-Hartman wrote: > > On Sun, Aug 07, 2022 at 02:25:33PM +0300, Oded Gabbay wrote: > > > 2. Common code to handle drivers that want to allow a single user at a > > > time to run open the device char file. > > > > Note, that's an impossible request, and one that the kernel should never > > worry about, so don't even try it. Think about userspace doing an call > > to dup() on an open char file descriptor and then passing that off > > somewhere else. > > Oded is talking about a model like VFIO has where the HW has a limited > number of concurrent state registers - lets say in this case the ASID > translation mapping the accelerator into DMA. Based on the number of drivers that I see submitted weekly that try to restrict their open call to just one user by using atomic variables or other tricks, I think my interpretation of this stands :) > Each 'struct file' that is created owns one of those HW state > registers, and each struct file is completely isolated from all > others. eg someone controlling the accelerator through struct file A > cannot access memory mapped into the accelerator through struct file > B. > > So, the number of struct files that can be created is capped at the > number of HW state registers the device can support (eg one for > Habana). > > This is different from the number of FDs pointing at the struct file. > Userpsace can open a HW state and point a lot of FDs at it, that is > userspace's problem. From a kernel view they all share one struct file > and thus one HW state. Yes, that's fine, if that is what is happening here, I have no objection. greg k-h
