block: Add qdev_prop_drive_iothread property type

Some qdev block devices have support for iothreads and take care of the
AioContext they are running in, but most devices don't know about any of
this. For the latter category, the qdev drive property must make sure
that their BlockBackend is in the main AioContext.

Unfortunately, while the current code just does the same thing for
devices that do support iothreads, this is not correct and it would show
as soon as we actually try to keep a consistent AioContext assignment
across all nodes and users of a block graph subtree: If a node is
already in a non-default AioContext because of one of its users,
attaching a new device should still be possible if that device can work
in the same AioContext. Switching the node back to the main context
first and only then into the device AioContext causes failure (because
the existing user wouldn't allow the switch to the main context).

So devices that support iothreads need a different kind of drive
property that leaves the node in its current AioContext, but by using
this type, the device promises to check later that it can work with this
context.

This patch adds the qdev infrastructure that allows devices to signal
that they handle iothreads and qdev should leave the AioContext alone.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 38 insertions, 5 deletions

diff --git a/hw/core/qdev-properties-system.c b/hw/core/qdev-properties-system.c
index 42e048f190..ba412dd2ca 100644
--- a/hw/core/qdev-properties-system.c
+++ b/hw/core/qdev-properties-system.c
@@ -69,8 +69,8 @@ static void set_pointer(Object *obj, Visitor *v, Property *prop,
 
 /* --- drive --- */
 
-static void parse_drive(DeviceState *dev, const char *str, void **ptr,
-                        const char *propname, Error **errp)
+static void do_parse_drive(DeviceState *dev, const char *str, void **ptr,
+                           const char *propname, bool iothread, Error **errp)
 {
     BlockBackend *blk;
     bool blk_created = false;
@@ -80,9 +80,16 @@ static void parse_drive(DeviceState *dev, const char *str, void **ptr,
     if (!blk) {
         BlockDriverState *bs = bdrv_lookup_bs(NULL, str, NULL);
         if (bs) {
-            /* BlockBackends of devices start in the main context and are only
-             * later moved into another context if the device supports that. */
-            blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
+            /*
+             * If the device supports iothreads, it will make sure to move the
+             * block node to the right AioContext if necessary (or fail if this
+             * isn't possible because of other users). Devices that are not
+             * aware of iothreads require their BlockBackends to be in the main
+             * AioContext.
+             */
+            AioContext *ctx = iothread ? bdrv_get_aio_context(bs) :
+                                         qemu_get_aio_context();
+            blk = blk_new(ctx, 0, BLK_PERM_ALL);
             blk_created = true;
 
             ret = blk_insert_bs(blk, bs, errp);
@@ -120,6 +127,18 @@ fail:
     }
 }
 
+static void parse_drive(DeviceState *dev, const char *str, void **ptr,
+                        const char *propname, Error **errp)
+{
+    do_parse_drive(dev, str, ptr, propname, false, errp);
+}
+
+static void parse_drive_iothread(DeviceState *dev, const char *str, void **ptr,
+                                 const char *propname, Error **errp)
+{
+    do_parse_drive(dev, str, ptr, propname, true, errp);
+}
+
 static void release_drive(Object *obj, const char *name, void *opaque)
 {
     DeviceState *dev = DEVICE(obj);
@@ -162,6 +181,12 @@ static void set_drive(Object *obj, Visitor *v, const char *name, void *opaque,
     set_pointer(obj, v, opaque, parse_drive, name, errp);
 }
 
+static void set_drive_iothread(Object *obj, Visitor *v, const char *name,
+                               void *opaque, Error **errp)
+{
+    set_pointer(obj, v, opaque, parse_drive_iothread, name, errp);
+}
+
 const PropertyInfo qdev_prop_drive = {
     .name  = "str",
     .description = "Node name or ID of a block device to use as a backend",
@@ -170,6 +195,14 @@ const PropertyInfo qdev_prop_drive = {
     .release = release_drive,
 };
 
+const PropertyInfo qdev_prop_drive_iothread = {
+    .name  = "str",
+    .description = "Node name or ID of a block device to use as a backend",
+    .get   = get_drive,
+    .set   = set_drive_iothread,
+    .release = release_drive,
+};
+
 /* --- character device --- */
 
 static void get_chr(Object *obj, Visitor *v, const char *name, void *opaque,