block/export: stop using is_external in vhost-user-blk server

vhost-user activity must be suspended during bdrv_drained_begin/end().
This prevents new requests from interfering with whatever is happening
in the drained section.

Previously this was done using aio_set_fd_handler()'s is_external
argument. In a multi-queue block layer world the aio_disable_external()
API cannot be used since multiple AioContext may be processing I/O, not
just one.

Switch to BlockDevOps->drained_begin/end() callbacks.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-8-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/export/vhost-user-blk-server.c

@@ -212,15 +212,21 @@ static void blk_aio_attached(AioContext *ctx, void *opaque)
 {
     VuBlkExport *vexp = opaque;
 
+    /*
+     * The actual attach will happen in vu_blk_drained_end() and we just
+     * restore ctx here.
+     */
     vexp->export.ctx = ctx;
-    vhost_user_server_attach_aio_context(&vexp->vu_server, ctx);
 }
 
 static void blk_aio_detach(void *opaque)
 {
     VuBlkExport *vexp = opaque;
 
-    vhost_user_server_detach_aio_context(&vexp->vu_server);
+    /*
+     * The actual detach already happened in vu_blk_drained_begin() but from
+     * this point on we must not access ctx anymore.
+     */
     vexp->export.ctx = NULL;
 }
 
@@ -272,6 +278,22 @@ static void vu_blk_exp_resize(void *opaque)
     vu_config_change_msg(&vexp->vu_server.vu_dev);
 }
 
+/* Called with vexp->export.ctx acquired */
+static void vu_blk_drained_begin(void *opaque)
+{
+    VuBlkExport *vexp = opaque;
+
+    vhost_user_server_detach_aio_context(&vexp->vu_server);
+}
+
+/* Called with vexp->export.blk AioContext acquired */
+static void vu_blk_drained_end(void *opaque)
+{
+    VuBlkExport *vexp = opaque;
+
+    vhost_user_server_attach_aio_context(&vexp->vu_server, vexp->export.ctx);
+}
+
 /*
  * Ensures that bdrv_drained_begin() waits until in-flight requests complete.
  *
@@ -285,6 +307,8 @@ static bool vu_blk_drained_poll(void *opaque)
 }
 
 static const BlockDevOps vu_blk_dev_ops = {
+    .drained_begin = vu_blk_drained_begin,
+    .drained_end   = vu_blk_drained_end,
     .drained_poll  = vu_blk_drained_poll,
     .resize_cb = vu_blk_exp_resize,
 };

util/vhost-user-server.c

@@ -278,7 +278,7 @@ set_watch(VuDev *vu_dev, int fd, int vu_evt,
         vu_fd_watch->fd = fd;
         vu_fd_watch->cb = cb;
         qemu_socket_set_nonblock(fd);
-        aio_set_fd_handler(server->ioc->ctx, fd, true, kick_handler,
+        aio_set_fd_handler(server->ioc->ctx, fd, false, kick_handler,
                            NULL, NULL, NULL, vu_fd_watch);
         vu_fd_watch->vu_dev = vu_dev;
         vu_fd_watch->pvt = pvt;
@@ -299,7 +299,7 @@ static void remove_watch(VuDev *vu_dev, int fd)
     if (!vu_fd_watch) {
         return;
     }
-    aio_set_fd_handler(server->ioc->ctx, fd, true,
+    aio_set_fd_handler(server->ioc->ctx, fd, false,
                        NULL, NULL, NULL, NULL, NULL);
 
     QTAILQ_REMOVE(&server->vu_fd_watches, vu_fd_watch, next);
@@ -362,7 +362,7 @@ void vhost_user_server_stop(VuServer *server)
         VuFdWatch *vu_fd_watch;
 
         QTAILQ_FOREACH(vu_fd_watch, &server->vu_fd_watches, next) {
-            aio_set_fd_handler(server->ctx, vu_fd_watch->fd, true,
+            aio_set_fd_handler(server->ctx, vu_fd_watch->fd, false,
                                NULL, NULL, NULL, NULL, vu_fd_watch);
         }
 
@@ -403,7 +403,7 @@ void vhost_user_server_attach_aio_context(VuServer *server, AioContext *ctx)
     qio_channel_attach_aio_context(server->ioc, ctx);
 
     QTAILQ_FOREACH(vu_fd_watch, &server->vu_fd_watches, next) {
-        aio_set_fd_handler(ctx, vu_fd_watch->fd, true, kick_handler, NULL,
+        aio_set_fd_handler(ctx, vu_fd_watch->fd, false, kick_handler, NULL,
                            NULL, NULL, vu_fd_watch);
     }
 
@@ -417,7 +417,7 @@ void vhost_user_server_detach_aio_context(VuServer *server)
         VuFdWatch *vu_fd_watch;
 
         QTAILQ_FOREACH(vu_fd_watch, &server->vu_fd_watches, next) {
-            aio_set_fd_handler(server->ctx, vu_fd_watch->fd, true,
+            aio_set_fd_handler(server->ctx, vu_fd_watch->fd, false,
                                NULL, NULL, NULL, NULL, vu_fd_watch);
         }