[RFC 2/3] aio-posix: enable IORING_SETUP_SINGLE_ISSUER

[Stefan Hajnoczi <stefanha@redhat.com>]

IORING_SETUP_SINGLE_ISSUER enables optimizations in the host Linux
kernel's io_uring code when the io_uring context is only used from a
single thread. This is true is QEMU because io_uring SQEs are submitted
from the same thread that processes the CQEs.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 util/fdmon-io_uring.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c
index 28b93c8ab9..4798439097 100644
--- a/util/fdmon-io_uring.c
+++ b/util/fdmon-io_uring.c
@@ -456,13 +456,30 @@ static const FDMonOps fdmon_io_uring_ops = {
     .add_sqe = fdmon_io_uring_add_sqe,
 };
 
+static inline bool is_creating_iothread(void)
+{
+    return qemu_get_thread_id() != getpid();
+}
+
 void fdmon_io_uring_setup(AioContext *ctx, Error **errp)
 {
     int ret;
+    /* TODO only enable these flags if they are available in the host's kernel headers */
+    unsigned flags = 0;
 
     ctx->io_uring_fd_tag = NULL;
 
-    ret = io_uring_queue_init(FDMON_IO_URING_ENTRIES, &ctx->fdmon_io_uring, 0);
+    /*
+     * The main thread's AioContexts are created from the main loop thread but
+     * may be accessed from multiple threads (e.g. vCPUs or the migration
+     * thread). IOThread AioContexts are only accessed from the IOThread
+     * itself.
+     */
+    if (is_creating_iothread()) {
+        flags = IORING_SETUP_SINGLE_ISSUER;
+    }
+
+    ret = io_uring_queue_init(FDMON_IO_URING_ENTRIES, &ctx->fdmon_io_uring, flags);
     if (ret != 0) {
         error_setg_errno(errp, -ret, "Failed to initialize io_uring");
         return;
-- 
2.50.1