aio: add non-blocking variant of aio_wait

This will be used when polling the GSource attached to an AioContext.
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

aio.c

@@ -93,13 +93,16 @@ void aio_set_event_notifier(AioContext *ctx,
                        (AioFlushHandler *)io_flush, notifier);
 }
 
-bool aio_wait(AioContext *ctx)
+bool aio_poll(AioContext *ctx, bool blocking)
 {
+    static struct timeval tv0;
     AioHandler *node;
     fd_set rdfds, wrfds;
     int max_fd = -1;
     int ret;
-    bool busy;
+    bool busy, progress;
+
+    progress = false;
 
     /*
      * If there are callbacks left that have been queued, we need to call then.
@@ -107,6 +110,11 @@ bool aio_wait(AioContext *ctx)
      * does not need a complete flush (as is the case for qemu_aio_wait loops).
      */
     if (aio_bh_poll(ctx)) {
+        blocking = false;
+        progress = true;
+    }
+
+    if (progress && !blocking) {
         return true;
     }
 
@@ -142,11 +150,11 @@ bool aio_wait(AioContext *ctx)
 
     /* No AIO operations?  Get us out of here */
     if (!busy) {
-        return false;
+        return progress;
     }
 
     /* wait until next event */
-    ret = select(max_fd, &rdfds, &wrfds, NULL, NULL);
+    ret = select(max_fd, &rdfds, &wrfds, NULL, blocking ? NULL : &tv0);
 
     /* if we have any readable fds, dispatch event */
     if (ret > 0) {
@@ -161,11 +169,13 @@ bool aio_wait(AioContext *ctx)
             if (!node->deleted &&
                 FD_ISSET(node->fd, &rdfds) &&
                 node->io_read) {
+                progress = true;
                 node->io_read(node->opaque);
             }
             if (!node->deleted &&
                 FD_ISSET(node->fd, &wrfds) &&
                 node->io_write) {
+                progress = true;
                 node->io_write(node->opaque);
             }
 
@@ -181,5 +191,5 @@ bool aio_wait(AioContext *ctx)
         }
     }
 
-    return true;
+    return progress;
 }

async.c

@@ -144,5 +144,5 @@ AioContext *aio_context_new(void)
 
 void aio_flush(AioContext *ctx)
 {
-    while (aio_wait(ctx));
+    while (aio_poll(ctx, true));
 }

main-loop.c

@@ -534,7 +534,7 @@ void qemu_aio_flush(void)
 
 bool qemu_aio_wait(void)
 {
-    return aio_wait(qemu_aio_context);
+    return aio_poll(qemu_aio_context, true);
 }
 
 void qemu_aio_set_fd_handler(int fd,

qemu-aio.h

@@ -133,13 +133,22 @@ void qemu_bh_delete(QEMUBH *bh);
  * outstanding AIO operations have been completed or cancelled. */
 void aio_flush(AioContext *ctx);
 
-/* Wait for a single AIO completion to occur.  This function will wait
- * until a single AIO event has completed and it will ensure something
- * has moved before returning. This can issue new pending aio as
- * result of executing I/O completion or bh callbacks.
+/* Progress in completing AIO work to occur.  This can issue new pending
+ * aio as a result of executing I/O completion or bh callbacks.
  *
- * Return whether there is still any pending AIO operation.  */
-bool aio_wait(AioContext *ctx);
+ * If there is no pending AIO operation or completion (bottom half),
+ * return false.  If there are pending bottom halves, return true.
+ *
+ * If there are no pending bottom halves, but there are pending AIO
+ * operations, it may not be possible to make any progress without
+ * blocking.  If @blocking is true, this function will wait until one
+ * or more AIO events have completed, to ensure something has moved
+ * before returning.
+ *
+ * If @blocking is false, this function will also return false if the
+ * function cannot make any progress without blocking.
+ */
+bool aio_poll(AioContext *ctx, bool blocking);
 
 #ifdef CONFIG_POSIX
 /* Returns 1 if there are still outstanding AIO requests; 0 otherwise */