1 files changed, 107 insertions, 0 deletions
diff --git a/util/fdmon-poll.c b/util/fdmon-poll.c
new file mode 100644
index 0000000000..488067b679
--- /dev/null
+++ b/util/fdmon-poll.c
@@ -0,0 +1,107 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * poll(2) file descriptor monitoring
+ *
+ * Uses ppoll(2) when available, g_poll() otherwise.
+ */
+
+#include "qemu/osdep.h"
+#include "aio-posix.h"
+#include "qemu/rcu_queue.h"
+
+/*
+ * These thread-local variables are used only in fdmon_poll_wait() around the
+ * call to the poll() system call.  In particular they are not used while
+ * aio_poll is performing callbacks, which makes it much easier to think about
+ * reentrancy!
+ *
+ * Stack-allocated arrays would be perfect but they have size limitations;
+ * heap allocation is expensive enough that we want to reuse arrays across
+ * calls to aio_poll().  And because poll() has to be called without holding
+ * any lock, the arrays cannot be stored in AioContext.  Thread-local data
+ * has none of the disadvantages of these three options.
+ */
+static __thread GPollFD *pollfds;
+static __thread AioHandler **nodes;
+static __thread unsigned npfd, nalloc;
+static __thread Notifier pollfds_cleanup_notifier;
+
+static void pollfds_cleanup(Notifier *n, void *unused)
+{
+    g_assert(npfd == 0);
+    g_free(pollfds);
+    g_free(nodes);
+    nalloc = 0;
+}
+
+static void add_pollfd(AioHandler *node)
+{
+    if (npfd == nalloc) {
+        if (nalloc == 0) {
+            pollfds_cleanup_notifier.notify = pollfds_cleanup;
+            qemu_thread_atexit_add(&pollfds_cleanup_notifier);
+            nalloc = 8;
+        } else {
+            g_assert(nalloc <= INT_MAX);
+            nalloc *= 2;
+        }
+        pollfds = g_renew(GPollFD, pollfds, nalloc);
+        nodes = g_renew(AioHandler *, nodes, nalloc);
+    }
+    nodes[npfd] = node;
+    pollfds[npfd] = (GPollFD) {
+        .fd = node->pfd.fd,
+        .events = node->pfd.events,
+    };
+    npfd++;
+}
+
+static int fdmon_poll_wait(AioContext *ctx, AioHandlerList *ready_list,
+                            int64_t timeout)
+{
+    AioHandler *node;
+    int ret;
+
+    assert(npfd == 0);
+
+    QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
+        if (!QLIST_IS_INSERTED(node, node_deleted) && node->pfd.events
+                && aio_node_check(ctx, node->is_external)) {
+            add_pollfd(node);
+        }
+    }
+
+    /* epoll(7) is faster above a certain number of fds */
+    if (fdmon_epoll_try_upgrade(ctx, npfd)) {
+        return ctx->fdmon_ops->wait(ctx, ready_list, timeout);
+    }
+
+    ret = qemu_poll_ns(pollfds, npfd, timeout);
+    if (ret > 0) {
+        int i;
+
+        for (i = 0; i < npfd; i++) {
+            int revents = pollfds[i].revents;
+
+            if (revents) {
+                aio_add_ready_handler(ready_list, nodes[i], revents);
+            }
+        }
+    }
+
+    npfd = 0;
+    return ret;
+}
+
+static void fdmon_poll_update(AioContext *ctx,
+                              AioHandler *old_node,
+                              AioHandler *new_node)
+{
+    /* Do nothing, AioHandler already contains the state we'll need */
+}
+
+const FDMonOps fdmon_poll_ops = {
+    .update = fdmon_poll_update,
+    .wait = fdmon_poll_wait,
+    .need_wait = aio_poll_disabled,
+};