Use io_uring_register_ring_fd() to skip fd operations

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>
Message-id: 20220531105011.111082-1-faithilikerun@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/io_uring.c

@@ -18,6 +18,7 @@
 #include "qapi/error.h"
 #include "trace.h"
 
+
 /* io_uring ring size */
 #define MAX_ENTRIES 128
 
@@ -434,8 +435,17 @@ LuringState *luring_init(Error **errp)
     }
 
     ioq_init(&s->io_q);
-    return s;
+#ifdef CONFIG_LIBURING_REGISTER_RING_FD
+    if (io_uring_register_ring_fd(&s->ring) < 0) {
+        /*
+         * Only warn about this error: we will fallback to the non-optimized
+         * io_uring operations.
+         */
+        warn_report("failed to register linux io_uring ring file descriptor");
+    }
+#endif
 
+    return s;
 }
 
 void luring_cleanup(LuringState *s)

meson.build

@@ -1752,6 +1752,7 @@ config_host_data.set('CONFIG_LIBNFS', libnfs.found())
 config_host_data.set('CONFIG_LIBSSH', libssh.found())
 config_host_data.set('CONFIG_LINUX_AIO', libaio.found())
 config_host_data.set('CONFIG_LINUX_IO_URING', linux_io_uring.found())
+config_host_data.set('CONFIG_LIBURING_REGISTER_RING_FD', cc.has_function('io_uring_register_ring_fd', prefix: '#include <liburing.h>', dependencies:linux_io_uring))
 config_host_data.set('CONFIG_LIBPMEM', libpmem.found())
 config_host_data.set('CONFIG_NUMA', numa.found())
 config_host_data.set('CONFIG_OPENGL', opengl.found())