From: David Woodhouse <dwmw@amazon.co.uk>

I can't see how this has ever worked. If I start with the simple attempt
"-nic user,model=xen", it creates a device with index -1 because it's
assuming that it'll be attached to a hubport. So it creates a frontend
at e.g. "/local/domain/84/device/vif/-1" and the guest fails to connect.

If I jump through hoops to give it a configuration that it might like:
    -netdev user,id=usernic
    -netdev hubport,hubid=0,id=hub0,netdev=usernic
    -nic,hubport,hubid=0,model=xen
... it *still* doesn't work. Qemu does actually use a slightly more
sensible index in the XenStore frontend path now, and the guest does
manage to connect to it. But on the Qemu side, the NIC still isn't
actually *attached* to the netdev:

qemu-system-x86_64: warning: hub port #net036 has no peer
qemu-system-x86_64: warning: hub 0 with no nics
qemu-system-x86_64: warning: netdev #net036 has no peer
qemu-system-x86_64: warning: requested NIC (anonymous, model xen) was not created (not supported by this machine?)

I can't see any point in the git history where the xen-nic driver
would actually look at that "handle" property, find the right netdev,
and actually *attach* the emulated NIC to anything.

Just rip out the special XenStore magic and instantiate a xen-net-device
on the XenBus. It all works now. Accept "model=xen-net-device" because
that's the actual Qemu device name and that's what works on HVM & emu.
Also accept model==NULL because why in $DEITY's name was that excluded
before anyway? What else are we doing to do for *PV* guests?

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 hw/xen/xen_devconfig.c              | 28 ++++++----------------------
 hw/xenpv/xen_machine_pv.c           | 13 ++++++++-----
 include/hw/xen/xen-legacy-backend.h |  2 +-
 3 files changed, 15 insertions(+), 28 deletions(-)

diff --git a/hw/xen/xen_devconfig.c b/hw/xen/xen_devconfig.c
index 3f77c675c6..d72bac38e7 100644
--- a/hw/xen/xen_devconfig.c
+++ b/hw/xen/xen_devconfig.c
@@ -1,4 +1,5 @@
 #include "qemu/osdep.h"
+#include "qapi/error.h"
 #include "hw/xen/xen-legacy-backend.h"
 #include "qemu/option.h"
 #include "sysemu/blockdev.h"
@@ -46,29 +47,12 @@ static int xen_config_dev_all(char *fe, char *be)
 
 /* ------------------------------------------------------------- */
 
-int xen_config_dev_nic(NICInfo *nic)
+int xen_config_dev_nic(BusState *xen_bus, NICInfo *nic)
 {
-    char fe[256], be[256];
-    char mac[20];
-    int vlan_id = -1;
-
-    net_hub_id_for_client(nic->netdev, &vlan_id);
-    snprintf(mac, sizeof(mac), "%02x:%02x:%02x:%02x:%02x:%02x",
-             nic->macaddr.a[0], nic->macaddr.a[1], nic->macaddr.a[2],
-             nic->macaddr.a[3], nic->macaddr.a[4], nic->macaddr.a[5]);
-    xen_pv_printf(NULL, 1, "config nic %d: mac=\"%s\"\n", vlan_id, mac);
-    xen_config_dev_dirs("vif", "qnic", vlan_id, fe, be, sizeof(fe));
-
-    /* frontend */
-    xenstore_write_int(fe, "handle",     vlan_id);
-    xenstore_write_str(fe, "mac",        mac);
-
-    /* backend */
-    xenstore_write_int(be, "handle",     vlan_id);
-    xenstore_write_str(be, "mac",        mac);
-
-    /* common stuff */
-    return xen_config_dev_all(fe, be);
+    DeviceState *dev = qdev_new("xen-net-device");
+    qdev_set_nic_properties(dev, nic);
+    qdev_realize_and_unref(dev, xen_bus, &error_fatal);
+    return 0;
 }
 
 int xen_config_dev_vfb(int vdev, const char *type)
diff --git a/hw/xenpv/xen_machine_pv.c b/hw/xenpv/xen_machine_pv.c
index 9f9f137f99..3d3a87e416 100644
--- a/hw/xenpv/xen_machine_pv.c
+++ b/hw/xenpv/xen_machine_pv.c
@@ -32,6 +32,7 @@
 
 static void xen_init_pv(MachineState *machine)
 {
+    BusState *xen_bus;
     int i;
 
     setup_xen_backend_ops();
@@ -62,15 +63,17 @@ static void xen_init_pv(MachineState *machine)
         vga_interface_created = true;
     }
 
+    xen_bus = xen_bus_init();
+
     /* configure nics */
     for (i = 0; i < nb_nics; i++) {
-        if (!nd_table[i].model || 0 != strcmp(nd_table[i].model, "xen"))
-            continue;
-        xen_config_dev_nic(nd_table + i);
+        if (!nd_table[i].model ||
+            g_str_equal(nd_table[i].model, "xen") ||
+            g_str_equal(nd_table[i].model, "xen-net-device")) {
+                xen_config_dev_nic(xen_bus, nd_table + i);
+            }
     }
 
-    xen_bus_init();
-
     /* config cleanup hook */
     atexit(xen_config_cleanup);
 }
diff --git a/include/hw/xen/xen-legacy-backend.h b/include/hw/xen/xen-legacy-backend.h
index fc42146bc2..01cab3a35e 100644
--- a/include/hw/xen/xen-legacy-backend.h
+++ b/include/hw/xen/xen-legacy-backend.h
@@ -81,7 +81,7 @@ extern struct XenDevOps xen_usb_ops;          /* xen-usb.c         */
 
 /* configuration (aka xenbus setup) */
 void xen_config_cleanup(void);
-int xen_config_dev_nic(NICInfo *nic);
+int xen_config_dev_nic(BusState *xen_bus, NICInfo *nic);
 int xen_config_dev_vfb(int vdev, const char *type);
 int xen_config_dev_vkbd(int vdev);
 int xen_config_dev_console(int vdev);
-- 
2.40.1