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 <stefa...@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
