On Mon, Jun 06, 2022 at 03:57:47PM +0100, Stefan Hajnoczi wrote: > Hi, > During Elena Afanasova's Outreachy project we discussed whether > ioregionfd should be a custom struct file_operations (anon inode) or a > userspace-provided file (socketpair, UNIX domain socket, etc). > Hello Stefan, > Back then it seemed more flexible and simpler to let userspace provide > the file. It may be worth revisiting this decision in light of the > recent io_uring IORING_OP_URING_CMD feature, which fits for this > performance-critical interface. > And Paolo was asking about io_uring in the review of the initial patches. > IORING_OP_URING_CMD involves a new struct file_operations->uring_cmd() > callback. It's a flexible userspace interface like ioctl(2) but designed > to be asynchronous. ioregionfd can provide a uring_cmd() that reads a > ioregionfd response from userspace and then writes the next ioregionfd > request to userspace. > > This single operation merges the request/response so only 1 syscall is > necessary per KVM MMIO/PIO exit instead of a read() + write(). Bypassing > the net/socket infrastructure is likely to help too. > > It would be interesting to benchmark this and compare it against the > existing userspace-provided file approach. Although it's not the same > scenario, results for the Linux NVMe driver using ->uring_cmd() are > promising: > https://www.snia.org/educational-library/enabling-asynchronous-i-o-passthru-nvme-native-applications-2021 > Yes, looks interesting, we were thinking about adding this. > The downside is it requires more code than general purpose I/O. In > addition to ->uring_cmd(), it's also worth implementing struct > file_operations read/write/poll so traditional file I/O syscalls work > for simple applications that don't want to use io_uring. > > It's possible to add ->uring_cmd() later but as a userspace developer I > would prefer the ->uring_cmd() approach, so I'm not sure it's worth > committing to the existing userspace-provided file approach? Makes total sense. I am going to start working on this and will come back with more questions. Thank you! Elena > > Stefan
