[libvirt] [PATCH RFC] libxl: reverse defaults on HVM net device attach

Joao Martins joao.m.martins at oracle.com
Fri Dec 9 11:35:37 UTC 2016

libvirt libxl picks its own default with respect to the default NIC
to use. libxlMakeNic is the one responsible for this and on boot it
picks LIBXL_NIC_TYPE_VIF_IOEMU such that it accomodates both PV and
emulated one. The good behaving guest at boot will then select the pv
and unplug the emulated device.

Now, on HVM when attaching an interface it will pick the same default
that is LIBXL_NIC_TYPE_VIF_IOEMU which as a result will fail the attach
(see xen commit 32e9d0f ("libxl: nic type defaults to vif in hotplug for
hvm guest"). Xen doesn't yet support the hotplug of emulated devices,
but we don't want to rule out that case either, which might get support
in the future. Hence we simply reverse the defaults when we are
attaching the interface which allows libvirt to prefer the PV nic first
without adding "model='netfront'" following the same pattern as above
commit. Also to avoid ruling out the emulated one we set to
LIBXL_NIC_TYPE_IOEMU when setting a model type that is not 'netfront'.

Signed-off-by: Joao Martins <joao.m.martins at oracle.com>

This allows Openstack to attach network interfaces, which currently
is broken on libxl if it doesn't set model 'netfront'. I am not sure
whether this is the best way (hence RFC) or if users should really be
setting the model='netfront' when attaching devices. But sounds to me
that it would be better to have by default the supported, and if users
want emulated attach to specify a nic model. Thoughts?
 src/libxl/libxl_conf.c   | 9 ++++++---
 src/libxl/libxl_conf.h   | 3 ++-
 src/libxl/libxl_driver.c | 2 +-
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/libxl/libxl_conf.c b/src/libxl/libxl_conf.c
index dcf8e7e..50aa958 100644
--- a/src/libxl/libxl_conf.c
+++ b/src/libxl/libxl_conf.c
@@ -881,9 +881,10 @@ libxlMakeDiskList(virDomainDefPtr def, libxl_domain_config *d_config)
 libxlMakeNic(virDomainDefPtr def,
              virDomainNetDefPtr l_nic,
-             libxl_device_nic *x_nic)
+             libxl_device_nic *x_nic,
+             bool attach)
-    bool ioemu_nic = def->os.type == VIR_DOMAIN_OSTYPE_HVM;
+    bool ioemu_nic = def->os.type == VIR_DOMAIN_OSTYPE_HVM && !attach;
     virDomainNetType actual_type = virDomainNetGetActualType(l_nic);
     virNetworkPtr network = NULL;
     virConnectPtr conn = NULL;
@@ -917,6 +918,8 @@ libxlMakeNic(virDomainDefPtr def,
             goto cleanup;
         if (STREQ(l_nic->model, "netfront"))
             x_nic->nictype = LIBXL_NIC_TYPE_VIF;
+        else
+            x_nic->nictype = LIBXL_NIC_TYPE_VIF_IOEMU;
     if (VIR_STRDUP(x_nic->ifname, l_nic->ifname) < 0)
@@ -1047,7 +1050,7 @@ libxlMakeNicList(virDomainDefPtr def,  libxl_domain_config *d_config)
         if (virDomainNetGetActualType(l_nics[i]) == VIR_DOMAIN_NET_TYPE_HOSTDEV)
-        if (libxlMakeNic(def, l_nics[i], &x_nics[nvnics]))
+        if (libxlMakeNic(def, l_nics[i], &x_nics[nvnics], false))
             goto error;
          * The devid (at least right now) will not get initialized by
diff --git a/src/libxl/libxl_conf.h b/src/libxl/libxl_conf.h
index 0ea76b4..851f3af 100644
--- a/src/libxl/libxl_conf.h
+++ b/src/libxl/libxl_conf.h
@@ -178,7 +178,8 @@ libxlMakeDisk(virDomainDiskDefPtr l_dev, libxl_device_disk *x_dev);
 libxlMakeNic(virDomainDefPtr def,
              virDomainNetDefPtr l_nic,
-             libxl_device_nic *x_nic);
+             libxl_device_nic *x_nic,
+             bool attach);
 libxlMakeVfb(virPortAllocatorPtr graphicsports,
              virDomainGraphicsDefPtr l_vfb, libxl_device_vfb *x_vfb);
diff --git a/src/libxl/libxl_driver.c b/src/libxl/libxl_driver.c
index b19b17e..7e5d9b6 100644
--- a/src/libxl/libxl_driver.c
+++ b/src/libxl/libxl_driver.c
@@ -3379,7 +3379,7 @@ libxlDomainAttachNetDevice(libxlDriverPrivatePtr driver,
         goto cleanup;
-    if (libxlMakeNic(vm->def, net, &nic) < 0)
+    if (libxlMakeNic(vm->def, net, &nic, true) < 0)
         goto cleanup;
     if (libxl_device_nic_add(cfg->ctx, vm->def->id, &nic, 0)) {

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