Difference between IORING_SETUP_REGISTERED_FD_ONLY and io_uring_register_ring_fd effects

[AomaYple]

I found that both seem to register file descriptors belonging to io_uring

[axboe]

If IORING_SETUP_REGISTERED_FD_ONLY is used, a regular file descriptor doesn't exist for the ring at all. If setup without it and then registered with io_uring_register_ring_fd(3), then the regular file descriptor still exists. If the ring fd is closed after that register ring fd call, then the final outcome is the same as having registered it with the setup flag.

[AomaYple]

If IORING_SETUP_REGISTERED_FD_ONLY is used, a regular file descriptor doesn't exist for the ring at all. If setup without it and then registered with io_uring_register_ring_fd(3), then the regular file descriptor still exists. If the ring fd is closed after that register ring fd call, then the final outcome is the same as having registered it with the setup flag.

So IORING_SETUP_REGISTERED_FD_ONLY is equivalent to using socket_direct as well, registering only a regular file descriptor index without a corresponding regular file descriptor.

io_uring_register_ring_fd is equivalent to registering a file descriptor index on a regular file descriptor.

Am I understanding this correctly?

If so, should subsequent use of the ring be accompanied by a flag like IOSQE_FIXED_FILE(just an analogy, not necessarily this one) indicating the use of a registered regular file descriptor index?

[axboe]

Registering the ring fd itself is indeed like registering normal files. But it doesn't go into the same table (that would cause a dependency loop), and hence you cannot use it for io_uring operations - it is only valid for io_uring syscalls, like io_uring_enter and io_uring_register. It achieves the same for syscalls, avoiding putting/getting of the file descriptor.

If so, should subsequent use of the ring be accompanied by a flag like IOSQE_FIXED_FILE(just an analogy, not necessarily this one) indicating the use of a registered regular file descriptor index?

Right, you need to pass in whether to use the registered ring fd to io_uring syscalls. To use it with io_uring_enter, you'd need to set IORING_ENTER_REGISTERED_RING in the flags passed in, and to use it with io_uring_register, you'd need to set IORING_REGISTER_USE_REGISTERED_RING. liburing hides these details, which is why you can just setup the ring as registered up front, or use the io_uring_register_ring_fd() helper after the fact.

For questions like this, just look at the liburing source code.

[AomaYple]

Registering the ring fd itself is indeed like registering normal files. But it doesn't go into the same table (that would cause a dependency loop), and hence you cannot use it for io_uring operations - it is only valid for io_uring syscalls, like io_uring_enter and io_uring_register. It achieves the same for syscalls, avoiding putting/getting of the file descriptor.

If so, should subsequent use of the ring be accompanied by a flag like IOSQE_FIXED_FILE(just an analogy, not necessarily this one) indicating the use of a registered regular file descriptor index?

Right, you need to pass in whether to use the registered ring fd to io_uring syscalls. To use it with io_uring_enter, you'd need to set IORING_ENTER_REGISTERED_RING in the flags passed in, and to use it with io_uring_register, you'd need to set IORING_REGISTER_USE_REGISTERED_RING. liburing hides these details, which is why you can just setup the ring as registered up front, or use the io_uring_register_ring_fd() helper after the fact.

For questions like this, just look at the liburing source code.

Oh, I looked at the source code and found that io_uring_register automatically uses the registered file descriptor based on INT_FLAG_REG_REG_RING.

As for __io_uring_submit, it uses __sys_io_uring_enter internally. Add the IORING_ENTER_REGISTERED_RING flag directly using ring_enter_flags. It seems to use this flag directly without determining whether the file descriptor is registered or not

[axboe]

Right, those are the flags that tell the io_uring syscalls to use a registered descriptor.