aio-posix: add io_uring fd monitoring implementation

The recent Linux io_uring API has several advantages over ppoll(2) and
epoll(2).  Details are given in the source code.

Add an io_uring implementation and make it the default on Linux.
Performance is the same as with epoll(7) but later patches add
optimizations that take advantage of io_uring.

It is necessary to change how aio_set_fd_handler() deals with deleting
AioHandlers since removing monitored file descriptors is asynchronous in
io_uring.  fdmon_io_uring_remove() marks the AioHandler deleted and
aio_set_fd_handler() will let it handle deletion in that case.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200305170806.1313245-6-stefanha@redhat.com
Message-Id: <20200305170806.1313245-6-stefanha@redhat.com>

1 files changed, 16 insertions, 4 deletions

diff --git a/util/aio-posix.c b/util/aio-posix.c
index 028b2abded..ffd9cc381b 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -57,10 +57,16 @@ static bool aio_remove_fd_handler(AioContext *ctx, AioHandler *node)
         g_source_remove_poll(&ctx->source, &node->pfd);
     }
 
+    node->pfd.revents = 0;
+
+    /* If the fd monitor has already marked it deleted, leave it alone */
+    if (QLIST_IS_INSERTED(node, node_deleted)) {
+        return false;
+    }
+
     /* If a read is in progress, just mark the node as deleted */
     if (qemu_lockcnt_count(&ctx->list_lock)) {
         QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted);
-        node->pfd.revents = 0;
         return false;
     }
     /* Otherwise, delete it for real.  We can't just mark it as
@@ -126,9 +132,6 @@ void aio_set_fd_handler(AioContext *ctx,
 
         QLIST_INSERT_HEAD_RCU(&ctx->aio_handlers, new_node, node);
     }
-    if (node) {
-        deleted = aio_remove_fd_handler(ctx, node);
-    }
 
     /* No need to order poll_disable_cnt writes against other updates;
      * the counter is only used to avoid wasting time and latency on
@@ -140,6 +143,9 @@ void aio_set_fd_handler(AioContext *ctx,
                atomic_read(&ctx->poll_disable_cnt) + poll_disable_change);
 
     ctx->fdmon_ops->update(ctx, node, new_node);
+    if (node) {
+        deleted = aio_remove_fd_handler(ctx, node);
+    }
     qemu_lockcnt_unlock(&ctx->list_lock);
     aio_notify(ctx);
 
@@ -565,11 +571,17 @@ void aio_context_setup(AioContext *ctx)
     ctx->fdmon_ops = &fdmon_poll_ops;
     ctx->epollfd = -1;
 
+    /* Use the fastest fd monitoring implementation if available */
+    if (fdmon_io_uring_setup(ctx)) {
+        return;
+    }
+
     fdmon_epoll_setup(ctx);
 }
 
 void aio_context_destroy(AioContext *ctx)
 {
+    fdmon_io_uring_destroy(ctx);
     fdmon_epoll_disable(ctx);
 }