io_uring: add a few test tools

This adds two test programs in tools/io_uring/ that demonstrate both
the raw io_uring API (and all features) through a small benchmark
app, io_uring-bench, and the liburing exposed API in a simplified
cp(1) implementation through io_uring-cp.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

tools/io_uring/queue.c

@@ -0,0 +1,164 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+
+#include "liburing.h"
+#include "barrier.h"
+
+static int __io_uring_get_completion(struct io_uring *ring,
+				     struct io_uring_cqe **cqe_ptr, int wait)
+{
+	struct io_uring_cq *cq = &ring->cq;
+	const unsigned mask = *cq->kring_mask;
+	unsigned head;
+	int ret;
+
+	*cqe_ptr = NULL;
+	head = *cq->khead;
+	do {
+		/*
+		 * It's necessary to use a read_barrier() before reading
+		 * the CQ tail, since the kernel updates it locklessly. The
+		 * kernel has the matching store barrier for the update. The
+		 * kernel also ensures that previous stores to CQEs are ordered
+		 * with the tail update.
+		 */
+		read_barrier();
+		if (head != *cq->ktail) {
+			*cqe_ptr = &cq->cqes[head & mask];
+			break;
+		}
+		if (!wait)
+			break;
+		ret = io_uring_enter(ring->ring_fd, 0, 1,
+					IORING_ENTER_GETEVENTS, NULL);
+		if (ret < 0)
+			return -errno;
+	} while (1);
+
+	if (*cqe_ptr) {
+		*cq->khead = head + 1;
+		/*
+		 * Ensure that the kernel sees our new head, the kernel has
+		 * the matching read barrier.
+		 */
+		write_barrier();
+	}
+
+	return 0;
+}
+
+/*
+ * Return an IO completion, if one is readily available
+ */
+int io_uring_get_completion(struct io_uring *ring,
+			    struct io_uring_cqe **cqe_ptr)
+{
+	return __io_uring_get_completion(ring, cqe_ptr, 0);
+}
+
+/*
+ * Return an IO completion, waiting for it if necessary
+ */
+int io_uring_wait_completion(struct io_uring *ring,
+			     struct io_uring_cqe **cqe_ptr)
+{
+	return __io_uring_get_completion(ring, cqe_ptr, 1);
+}
+
+/*
+ * Submit sqes acquired from io_uring_get_sqe() to the kernel.
+ *
+ * Returns number of sqes submitted
+ */
+int io_uring_submit(struct io_uring *ring)
+{
+	struct io_uring_sq *sq = &ring->sq;
+	const unsigned mask = *sq->kring_mask;
+	unsigned ktail, ktail_next, submitted;
+	int ret;
+
+	/*
+	 * If we have pending IO in the kring, submit it first. We need a
+	 * read barrier here to match the kernels store barrier when updating
+	 * the SQ head.
+	 */
+	read_barrier();
+	if (*sq->khead != *sq->ktail) {
+		submitted = *sq->kring_entries;
+		goto submit;
+	}
+
+	if (sq->sqe_head == sq->sqe_tail)
+		return 0;
+
+	/*
+	 * Fill in sqes that we have queued up, adding them to the kernel ring
+	 */
+	submitted = 0;
+	ktail = ktail_next = *sq->ktail;
+	while (sq->sqe_head < sq->sqe_tail) {
+		ktail_next++;
+		read_barrier();
+
+		sq->array[ktail & mask] = sq->sqe_head & mask;
+		ktail = ktail_next;
+
+		sq->sqe_head++;
+		submitted++;
+	}
+
+	if (!submitted)
+		return 0;
+
+	if (*sq->ktail != ktail) {
+		/*
+		 * First write barrier ensures that the SQE stores are updated
+		 * with the tail update. This is needed so that the kernel
+		 * will never see a tail update without the preceeding sQE
+		 * stores being done.
+		 */
+		write_barrier();
+		*sq->ktail = ktail;
+		/*
+		 * The kernel has the matching read barrier for reading the
+		 * SQ tail.
+		 */
+		write_barrier();
+	}
+
+submit:
+	ret = io_uring_enter(ring->ring_fd, submitted, 0,
+				IORING_ENTER_GETEVENTS, NULL);
+	if (ret < 0)
+		return -errno;
+
+	return 0;
+}
+
+/*
+ * Return an sqe to fill. Application must later call io_uring_submit()
+ * when it's ready to tell the kernel about it. The caller may call this
+ * function multiple times before calling io_uring_submit().
+ *
+ * Returns a vacant sqe, or NULL if we're full.
+ */
+struct io_uring_sqe *io_uring_get_sqe(struct io_uring *ring)
+{
+	struct io_uring_sq *sq = &ring->sq;
+	unsigned next = sq->sqe_tail + 1;
+	struct io_uring_sqe *sqe;
+
+	/*
+	 * All sqes are used
+	 */
+	if (next - sq->sqe_head > *sq->kring_entries)
+		return NULL;
+
+	sqe = &sq->sqes[sq->sqe_tail & *sq->kring_mask];
+	sq->sqe_tail = next;
+	return sqe;
+}