[PATCH v3] Use io_uring_register_ring_fd() to skip fd operations

[Sam Li <faithilikerun@gmail.com>]

Linux recently added a new io_uring(7) optimization API that QEMU
doesn't take advantage of yet. The liburing library that QEMU uses
has added a corresponding new API calling io_uring_register_ring_fd().
When this API is called after creating the ring, the io_uring_submit()
library function passes a flag to the io_uring_enter(2) syscall
allowing it to skip the ring file descriptor fdget()/fdput()
operations. This saves some CPU cycles.

Signed-off-by: Sam Li <faithilikerun@gmail.com>
---
 block/io_uring.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/block/io_uring.c b/block/io_uring.c
index 782afdb433..51f4834b69 100644
--- a/block/io_uring.c
+++ b/block/io_uring.c
@@ -435,8 +435,16 @@ LuringState *luring_init(Error **errp)
     }
 
     ioq_init(&s->io_q);
-    return s;
+    if (io_uring_register_ring_fd(&s->ring) < 0) {
+        /*
+         * Only warn about this error: we will fallback to the non-optimized
+         * io_uring operations.
+         */
+        error_setg_errno(errp, errno,
+                         "failed to register linux io_uring ring file descriptor");
+    }
 
+    return s;
 }
 
 void luring_cleanup(LuringState *s)
-- 
2.35.1