[PATCH 01/11] qdev/qbus: add hidden device support

[Jens Freimann <jfreimann@redhat.com>]

This adds support for hiding a device to the qbus and qdev APIs.  The
first user of this will be the virtio-net failover feature but the API
introduced with this patch could be used to implement other features as
well, for example hiding pci devices when a pci bus is powered off.

qdev_device_add() is modified to check for a net_failover_pair_id
argument in the option string. A DeviceListener callback
should_be_hidden() is added. It can be used by a standby device to
inform qdev that this device should not be added now. The standby device
handler can store the device options to plug the device in at a later
point in time.

One reason for hiding the device is that we don't want to expose both
devices to the guest kernel until the respective virtio feature bit
VIRTIO_NET_F_STANDBY was negotiated and we know that the devices will be
handled correctly by the guest.

More information on the kernel feature this is using:
 https://www.kernel.org/doc/html/latest/networking/net_failover.html

An example where the primary device is a vfio-pci device and the standby
device is a virtio-net device:

A device is hidden when it has an "net_failover_pair_id" option, e.g.

 -device virtio-net-pci,...,failover=on,...
 -device vfio-pci,...,net_failover_pair_id=net1,...

Signed-off-by: Jens Freimann <jfreimann@redhat.com>
---
 hw/core/qdev.c         | 23 +++++++++++++++++++++++
 include/hw/qdev-core.h |  8 ++++++++
 qdev-monitor.c         | 36 +++++++++++++++++++++++++++++++++---
 vl.c                   |  6 ++++--
 4 files changed, 68 insertions(+), 5 deletions(-)

diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index cbad6c1d55..89c134ec53 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -212,6 +212,29 @@ void device_listener_unregister(DeviceListener *listener)
     QTAILQ_REMOVE(&device_listeners, listener, link);
 }
 
+bool qdev_should_hide_device(QemuOpts *opts)
+{
+    int rc;
+    DeviceListener *listener;
+
+    QTAILQ_FOREACH(listener, &device_listeners, link) {
+       if (listener->should_be_hidden) {
+            /* should_be_hidden_will return
+             *  1 if device matches opts and it should be hidden
+             *  0 if device matches opts and should not be hidden
+             *  -1 if device doesn't match ops
+             */
+            rc = listener->should_be_hidden(listener, opts);
+        }
+
+        if (rc > 0) {
+            break;
+        }
+    }
+
+    return rc > 0;
+}
+
 void qdev_set_legacy_instance_id(DeviceState *dev, int alias_id,
                                  int required_for_version)
 {
diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index aa123f88cb..28f594a47d 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -154,6 +154,12 @@ struct DeviceState {
 struct DeviceListener {
     void (*realize)(DeviceListener *listener, DeviceState *dev);
     void (*unrealize)(DeviceListener *listener, DeviceState *dev);
+    /*
+     * This callback is called upon init of the DeviceState and allows to
+     * inform qdev that a device should be hidden, depending on the device
+     * opts, for example, to hide a standby device.
+     */
+    int (*should_be_hidden)(DeviceListener *listener, QemuOpts *device_opts);
     QTAILQ_ENTRY(DeviceListener) link;
 };
 
@@ -451,4 +457,6 @@ static inline bool qbus_is_hotpluggable(BusState *bus)
 void device_listener_register(DeviceListener *listener);
 void device_listener_unregister(DeviceListener *listener);
 
+bool qdev_should_hide_device(QemuOpts *opts);
+
 #endif
diff --git a/qdev-monitor.c b/qdev-monitor.c
index 148df9cacf..508d85df87 100644
--- a/qdev-monitor.c
+++ b/qdev-monitor.c
@@ -32,9 +32,11 @@
 #include "qemu/help_option.h"
 #include "qemu/option.h"
 #include "qemu/qemu-print.h"
+#include "qemu/option_int.h"
 #include "sysemu/block-backend.h"
 #include "sysemu/sysemu.h"
 #include "migration/misc.h"
+#include "migration/migration.h"
 
 /*
  * Aliases were a bad idea from the start.  Let's keep them
@@ -562,14 +564,40 @@ void qdev_set_id(DeviceState *dev, const char *id)
     }
 }
 
+static int is_failover_device(void *opaque, const char *name, const char *value,
+                        Error **errp)
+{
+    if (strcmp(name, "net_failover_pair_id") == 0) {
+        QemuOpts *opts = (QemuOpts *)opaque;
+
+        if (qdev_should_hide_device(opts)) {
+            return 1;
+        }
+    }
+
+    return 0;
+}
+
+static bool should_hide_device(QemuOpts *opts)
+{
+    if (qemu_opt_foreach(opts, is_failover_device, opts, NULL) == 0) {
+        return false;
+    }
+    return true;
+}
+
 DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
 {
     DeviceClass *dc;
     const char *driver, *path;
-    DeviceState *dev;
+    DeviceState *dev = NULL;
     BusState *bus = NULL;
     Error *err = NULL;
 
+    if (opts && should_hide_device(opts)) {
+        return NULL;
+    }
+
     driver = qemu_opt_get(opts, "driver");
     if (!driver) {
         error_setg(errp, QERR_MISSING_PARAMETER, "driver");
@@ -648,8 +676,10 @@ DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
 
 err_del_dev:
     error_propagate(errp, err);
-    object_unparent(OBJECT(dev));
-    object_unref(OBJECT(dev));
+    if (dev) {
+        object_unparent(OBJECT(dev));
+        object_unref(OBJECT(dev));
+    }
     return NULL;
 }
 
diff --git a/vl.c b/vl.c
index 4489cfb2bb..62c388cb49 100644
--- a/vl.c
+++ b/vl.c
@@ -2204,10 +2204,12 @@ static int device_init_func(void *opaque, QemuOpts *opts, Error **errp)
     DeviceState *dev;
 
     dev = qdev_device_add(opts, errp);
-    if (!dev) {
+    if (!dev && *errp) {
+        error_report_err(*errp);
         return -1;
+    } else if (dev) {
+        object_unref(OBJECT(dev));
     }
-    object_unref(OBJECT(dev));
     return 0;
 }
 
-- 
2.21.0