Add XML config switch to enable/disable vhost-net support

This patch is in response to

  https://bugzilla.redhat.com/show_bug.cgi?id=643050

The existing libvirt support for the vhost-net backend to the virtio
network driver happens automatically - if the vhost-net device is
available, it is always enabled, otherwise the standard userland
virtio backend is used.

This patch makes it possible to force whether or not vhost-net is used
with a bit of XML. Adding a <driver> element to the interface XML, eg:

     <interface type="network">
       <model type="virtio"/>
       <driver name="vhost"/>

will force use of vhost-net (if it's not available, the domain will
fail to start). if driver name="qemu", vhost-net will not be used even
if it is available.

If there is no <driver name='xxx'/> in the config, libvirt will revert
to the pre-existing automatic behavior - use vhost-net if it's
available, and userland backend if vhost-net isn't available.

docs/schemas/domain.rng

@@ -1004,6 +1004,19 @@
           <empty/>
         </element>
       </optional>
+      <optional>
+        <element name="driver">
+          <optional>
+            <attribute name="name">
+              <choice>
+                <value>qemu</value>
+                <value>vhost</value>
+              </choice>
+            </attribute>
+          </optional>
+          <empty/>
+        </element>
+      </optional>
       <optional>
         <ref name="address"/>
       </optional>

src/conf/domain_conf.c

@@ -185,6 +185,11 @@ VIR_ENUM_IMPL(virDomainNet, VIR_DOMAIN_NET_TYPE_LAST,
               "internal",
               "direct")
 
+VIR_ENUM_IMPL(virDomainNetBackend, VIR_DOMAIN_NET_BACKEND_TYPE_LAST,
+              "default",
+              "qemu",
+              "vhost")
+
 VIR_ENUM_IMPL(virDomainChrChannelTarget,
               VIR_DOMAIN_CHR_CHANNEL_TARGET_TYPE_LAST,
               "guestfwd",
@@ -2304,6 +2309,7 @@ virDomainNetDefParseXML(virCapsPtr caps,
     char *address = NULL;
     char *port = NULL;
     char *model = NULL;
+    char *backend = NULL;
     char *filter = NULL;
     char *internal = NULL;
     char *devaddr = NULL;
@@ -2386,6 +2392,8 @@ virDomainNetDefParseXML(virCapsPtr caps,
                 script = virXMLPropString(cur, "path");
             } else if (xmlStrEqual (cur->name, BAD_CAST "model")) {
                 model = virXMLPropString(cur, "type");
+            } else if (xmlStrEqual (cur->name, BAD_CAST "driver")) {
+                backend = virXMLPropString(cur, "name");
             } else if (xmlStrEqual (cur->name, BAD_CAST "filterref")) {
                 filter = virXMLPropString(cur, "filter");
                 VIR_FREE(filterparams);
@@ -2573,6 +2581,19 @@ virDomainNetDefParseXML(virCapsPtr caps,
         model = NULL;
     }
 
+    if ((backend != NULL) &&
+        (def->model && STREQ(def->model, "virtio"))) {
+        int b;
+        if (((b = virDomainNetBackendTypeFromString(backend)) < 0) ||
+            (b == VIR_DOMAIN_NET_BACKEND_TYPE_DEFAULT)) {
+            virDomainReportError(VIR_ERR_INTERNAL_ERROR,
+                                 _("Unknown interface <driver name='%s'> "
+                                   "has been specified"),
+                                 backend);
+            goto error;
+        }
+        def->backend = b;
+    }
     if (filter != NULL) {
         switch (def->type) {
         case VIR_DOMAIN_NET_TYPE_ETHERNET:
@@ -2599,6 +2620,7 @@ cleanup:
     VIR_FREE(script);
     VIR_FREE(bridge);
     VIR_FREE(model);
+    VIR_FREE(backend);
     VIR_FREE(filter);
     VIR_FREE(type);
     VIR_FREE(internal);
@@ -6318,9 +6340,14 @@ virDomainNetDefFormat(virBufferPtr buf,
     if (def->ifname)
         virBufferEscapeString(buf, "      <target dev='%s'/>\n",
                               def->ifname);
-    if (def->model)
+    if (def->model) {
         virBufferEscapeString(buf, "      <model type='%s'/>\n",
                               def->model);
+        if (STREQ(def->model, "virtio") && def->backend) {
+            virBufferVSprintf(buf, "      <driver name='%s'/>\n",
+                              virDomainNetBackendTypeToString(def->backend));
+        }
+    }
     if (def->filter) {
         virBufferEscapeString(buf, "      <filterref filter='%s'",
                               def->filter);

src/conf/domain_conf.h

@@ -292,6 +292,14 @@ enum virDomainNetType {
     VIR_DOMAIN_NET_TYPE_LAST,
 };
 
+/* the backend driver used for virtio interfaces */
+enum virDomainNetBackendType {
+    VIR_DOMAIN_NET_BACKEND_TYPE_DEFAULT, /* prefer kernel, fall back to user */
+    VIR_DOMAIN_NET_BACKEND_TYPE_QEMU,    /* userland */
+    VIR_DOMAIN_NET_BACKEND_TYPE_VHOST,   /* kernel */
+
+    VIR_DOMAIN_NET_BACKEND_TYPE_LAST,
+};
 
 /* the mode type for macvtap devices */
 enum virDomainNetdevMacvtapType {
@@ -310,6 +318,7 @@ struct _virDomainNetDef {
     enum virDomainNetType type;
     unsigned char mac[VIR_MAC_BUFLEN];
     char *model;
+    enum virDomainNetBackendType backend;
     union {
         struct {
             char *dev;
@@ -1273,6 +1282,7 @@ VIR_ENUM_DECL(virDomainControllerModel)
 VIR_ENUM_DECL(virDomainFS)
 VIR_ENUM_DECL(virDomainFSAccessMode)
 VIR_ENUM_DECL(virDomainNet)
+VIR_ENUM_DECL(virDomainNetBackend)
 VIR_ENUM_DECL(virDomainChrDevice)
 VIR_ENUM_DECL(virDomainChrChannelTarget)
 VIR_ENUM_DECL(virDomainChrConsoleTarget)

src/qemu/qemu_command.c

@@ -302,24 +302,58 @@ cleanup:
 }
 
 
-int
+static int
 qemuOpenVhostNet(virDomainNetDefPtr net,
-                 unsigned long long qemuCmdFlags)
+                 unsigned long long qemuCmdFlags,
+                 int *vhostfd)
 {
 
-    /* If qemu supports vhost-net mode (including the -netdev command
+    *vhostfd = -1;   /* assume we won't use vhost */
-     * option), the nic model is virtio, and we can open
-     * /dev/vhost_net, assume that vhost-net mode is available and
-     * return the fd to /dev/vhost_net. Otherwise, return -1.
-     */
 
+    /* If the config says explicitly to not use vhost, return now */
+    if (net->backend == VIR_DOMAIN_NET_BACKEND_TYPE_QEMU) {
+       return 0;
+    }
+
+    /* If qemu doesn't support vhost-net mode (including the -netdev command
+     * option), don't try to open the device.
+     */
     if (!(qemuCmdFlags & QEMUD_CMD_FLAG_VNET_HOST &&
           qemuCmdFlags & QEMUD_CMD_FLAG_NETDEV &&
-          qemuCmdFlags & QEMUD_CMD_FLAG_DEVICE &&
+          qemuCmdFlags & QEMUD_CMD_FLAG_DEVICE)) {
-          net->model && STREQ(net->model, "virtio")))
+        if (net->backend == VIR_DOMAIN_NET_BACKEND_TYPE_VHOST) {
+            qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                            "%s", _("vhost-net is not supported with "
+                                    "this QEMU binary"));
             return -1;
+        }
+        return 0;
+    }
 
-    return open("/dev/vhost-net", O_RDWR, 0);
+    /* If the nic model isn't virtio, don't try to open. */
+    if (!(net->model && STREQ(net->model, "virtio"))) {
+        if (net->backend == VIR_DOMAIN_NET_BACKEND_TYPE_VHOST) {
+            qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                            "%s", _("vhost-net is only supported for "
+                                    "virtio network interfaces"));
+            return -1;
+        }
+        return 0;
+    }
+
+    *vhostfd = open("/dev/vhost-net", O_RDWR);
+
+    /* If the config says explicitly to use vhost and we couldn't open it,
+     * report an error.
+     */
+    if ((*vhostfd < 0) &&
+        (net->backend == VIR_DOMAIN_NET_BACKEND_TYPE_VHOST)) {
+        qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                        "%s", _("vhost-net was requested for an interface, "
+                                "but is unavailable"));
+        return -1;
+    }
+    return 0;
 }
 
 
@@ -3281,7 +3315,10 @@ qemuBuildCommandLine(virConnectPtr conn,
                 net->type == VIR_DOMAIN_NET_TYPE_DIRECT) {
                 /* Attempt to use vhost-net mode for these types of
                    network device */
-                int vhostfd = qemuOpenVhostNet(net, qemuCmdFlags);
+                int vhostfd;
+
+                if (qemuOpenVhostNet(net, qemuCmdFlags, &vhostfd) < 0)
+                    goto error;
                 if (vhostfd >= 0) {
                     virCommandTransferFD(cmd, vhostfd);
 
@@ -4626,6 +4663,12 @@ qemuParseCommandLineNet(virCapsPtr caps,
         } else if (STREQ(keywords[i], "model")) {
             def->model = values[i];
             values[i] = NULL;
+        } else if (STREQ(keywords[i], "vhost")) {
+            if ((values[i] == NULL) || STREQ(values[i], "on")) {
+                def->backend = VIR_DOMAIN_NET_BACKEND_TYPE_VHOST;
+            } else if (STREQ(keywords[i], "off")) {
+                def->backend = VIR_DOMAIN_NET_BACKEND_TYPE_QEMU;
+            }
         }
     }
 

src/qemu/qemu_command.h

@@ -111,9 +111,6 @@ int qemuNetworkIfaceConnect(virConnectPtr conn,
                             unsigned long long qemCmdFlags)
     ATTRIBUTE_NONNULL(1);
 
-int qemuOpenVhostNet(virDomainNetDefPtr net,
-                     unsigned long long qemuCmdFlags);
-
 int qemuPhysIfaceConnect(virConnectPtr conn,
                          struct qemud_driver *driver,
                          virDomainNetDefPtr net,