[PATCH] add a helper function to verify io_uring functionality

[Glauber Costa <glauber@scylladb.com>]

It is common for an application using an ever-evolving interface to want
to inquire about the presence of certain functionality it plans to use.

The boilerplate to do that is about always the same: find places that
have feature bits, match that with what we need, rinse, repeat.
Therefore it makes sense to move this to a library function.

We have two places in which we can check for such features: the feature
flag returned by io_uring_init_params(), and the resulting array
returning from io_uring_probe.

I tried my best to communicate as well as possible in the function
signature the fact that this is not supposed to test the availability
of io_uring (which is straightforward enough), but rather a minimum set
of requirements for usage.

Signed-off-by: Glauber Costa <glauber@scylladb.com>
CC: Avi Kivity <avi@scylladb.com>
---
 src/include/liburing.h | 13 +++++++++++++
 src/liburing.map       |  1 +
 src/setup.c            | 39 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+)

diff --git a/src/include/liburing.h b/src/include/liburing.h
index 83d11dd..d740083 100644
--- a/src/include/liburing.h
+++ b/src/include/liburing.h
@@ -72,6 +72,19 @@ struct io_uring {
 /*
  * Library interface
  */
+
+/* Checks that io_uring is modern enough for a particular case.
+ * Check it by verifying that:
+ *
+ *  - io_uring is available
+ *  - the io_uring_probe call is available, so opcodes can be checked
+ *  - all opcodes the application wants to use are supported
+ *  - the features requested are present.
+ *
+ *  return 0 if io_uring is not usable, 1 otherwise.
+ */
+extern int io_uring_check_minimum_support(const int* operations, int noperations, int features);
+
 extern int io_uring_queue_init_params(unsigned entries, struct io_uring *ring,
 	struct io_uring_params *p);
 extern int io_uring_queue_init(unsigned entries, struct io_uring *ring,
diff --git a/src/liburing.map b/src/liburing.map
index b45f373..579d4de 100644
--- a/src/liburing.map
+++ b/src/liburing.map
@@ -72,4 +72,5 @@ LIBURING_0.4 {
 		io_uring_register_probe;
 		io_uring_register_personality;
 		io_uring_unregister_personality;
+		io_uring_check_minimum_support;
 } LIBURING_0.3;
diff --git a/src/setup.c b/src/setup.c
index c53f234..7e46219 100644
--- a/src/setup.c
+++ b/src/setup.c
@@ -4,6 +4,7 @@
 #include <unistd.h>
 #include <errno.h>
 #include <string.h>
+#include <stdlib.h>
 
 #include "liburing/compat.h"
 #include "liburing/io_uring.h"
@@ -167,3 +168,41 @@ void io_uring_queue_exit(struct io_uring *ring)
 	io_uring_unmap_rings(sq, cq);
 	close(ring->ring_fd);
 }
+
+int io_uring_check_minimum_support(const int* operations, int noperations, int features)
+{
+	struct io_uring_params p;
+	struct io_uring_probe* probe;
+	struct io_uring ring;
+	int r;
+	int i;
+	int ret = 0;
+
+	memset(&p, 0, sizeof(p));
+	r = io_uring_queue_init_params(2, &ring, &p);
+	if (r < 0)
+		return ret;
+
+	if ((p.features & features) != features)
+		goto exit;
+
+	size_t len = sizeof(*probe) + 256 * sizeof(struct io_uring_probe_op);
+	probe = malloc(len);
+	memset(probe, 0, len);
+	r = io_uring_register_probe(&ring, probe, 256);
+	if (r < 0)
+		goto exit;
+
+	for (i = 0; i < noperations; i++) {
+		int op = operations[i];
+		if (probe->last_op < op)
+			goto exit;
+
+		if (!(probe->ops[op].flags & IO_URING_OP_SUPPORTED))
+			goto exit;
+	}
+	ret = 1;
+exit:
+	io_uring_queue_exit(&ring);
+	return ret;
+}
-- 
2.20.1