Normally MSG_RING requires both a source and a destination ring. But
some users don't always have a ring avilable to send a message from, yet
they still need to notify a target ring.
Add support for using io_uring_register(2) without having a source ring,
using a file descriptor of -1 for that. Internally those are called
blind registration opcodes. Implement IORING_REGISTER_SEND_MSG_RING as a
blind opcode, which simply takes an sqe that the application can put on
the stack and use the normal liburing helpers to initialize it. Then the
app can call:
io_uring_register(-1, IORING_REGISTER_SEND_MSG_RING, &sqe, 1);
and get the same behavior in terms of the target, where a CQE is posted
with the details given in the sqe.
For now this takes a single sqe pointer argument, and hence arg must
be set to that, and nr_args must be 1. Could easily be extended to take
an array of sqes, but for now let's keep it simple.
Link: https://lore.kernel.org/r/20240924115932.116167-3-axboe@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>
With slab accounting, allocating and freeing memory has considerable
overhead. Add a basic alloc cache for the io_kiocb allocations that
msg_ring needs to do. Unlike other caches, this one is used by the
sender, grabbing it from the remote ring. When the remote ring gets
the posted completion, it'll free it locally. Hence it is separately
locked, using ctx->msg_lock.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
