The io_uring interface is preferable to classic syscalls for any kind of syscall that requires the kernel to perform a significant amount of work or to wait for some external events, so a long time could pass until the completion of the work or until the occurrence of the external events, and possibly for some auxiliary syscalls that modify settings relevant to the previous syscalls.

Many of the most important syscalls belong to this class, but most of them already have io_uring alternatives. Many other syscalls are not of this kind, so for them there is no reason to replace the traditional syscall interface.

For the syscalls appropriate for io_uring the essential characteristic is that their completions can occur in a different order than the submissions of their requests, which is why all previous methods of handling them were inefficient, because they required the user program to waste time with superfluous activities, instead of just picking the completion messages from a queue.

Where this out-of-order execution is not needed, invoking the syscalls in the same way as you invoke any user function is fine.

The initial implementation of io_uring lacked the alternatives for many important syscalls, but since then most of the holes have been filled.

Besides more obvious syscalls of the read/write type, which are used for files accessed in the classic way, in order to reach maximum performance with memory-mapped files the old "madvise" syscall must be replaced with IORING_OP_MADVISE, used with Linux-specific advices (MADV_COLD, MADV_PAGEOUT, MADV_POPULATE_READ, MADV_POPULATE_WRITE).