[RFC QEMU PATCH 02/10] qdev/qbus: Allow hidden devices to be busless on QEMU startup

[Joshua Lant <joshualant@gmail.com>]

Extend the capability for hiding devices, introduced for virtio-net
device in:

commit f3a8505656935cde32e28c1c6317f725084da1e0
Author: Jens Freimann <jfreimann@redhat.com>
Date:   Tue Oct 29 12:48:55 2019 +0100
qdev/qbus: add hidden device support

Currently only endpoint devices can be hidden with a primary device
and failover (known static configuration). However, looking at future
composable systems, we see a need for hidden devices which have no associated
bus upon boot. Move the check for hidden devices to before the bus
search, and if it is hidden ignore the case where the device was
described on the CLI without the "bus=" field.

This is motivated by a specific use-case: implementing the VCS
command set, part of the CXL specification (CXL r3.2 Section 7.1.3). In
this scenario a switch controlled by a Fabric Manager is able to change the
virtual hierarchy of devices seen by a guest within a fixed physical system
topology. The connecting bus is not known until runtime when
a bind command is issued by the Fabric Manager.

Signed-off-by: Joshua Lant <joshualant@gmail.com>
---
 system/qdev-monitor.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
index f2aa400a77..b51dfe0645 100644
--- a/system/qdev-monitor.c
+++ b/system/qdev-monitor.c
@@ -650,6 +650,7 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
     DeviceState *dev;
     BusState *bus = NULL;
     QDict *properties;
+    bool hide_device;
 
     driver = qdict_get_try_str(opts, "driver");
     if (!driver) {
@@ -663,6 +664,11 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
         return NULL;
     }
 
+    /* Is the device hidden from the guest?
+     * If yes, no need to find a default bus if none given...
+     * Bus could be provided at runtime (i.e. in a switch)*/
+    hide_device = qdev_should_hide_device(opts, from_json, errp);
+
     /* find bus */
     path = qdict_get_try_str(opts, "bus");
     if (path != NULL) {
@@ -675,14 +681,14 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
                        driver, object_get_typename(OBJECT(bus)));
             return NULL;
         }
-    } else if (dc->bus_type != NULL) {
+    } else if (dc->bus_type != NULL && !hide_device) {
         bus = qdev_find_default_bus(dc, errp);
         if (!bus) {
             return NULL;
         }
     }
 
-    if (qdev_should_hide_device(opts, from_json, errp)) {
+    if (hide_device) {
         if (bus && !qbus_is_hotpluggable(bus)) {
             error_setg(errp, "Bus '%s' does not support hotplugging",
                        bus->name);
-- 
2.47.3