cpu: Allow fine tuning of "host-model" cpu

https://bugzilla.redhat.com/show_bug.cgi?id=799354

Until now, the "host-model" cpu mode couldn't be influenced. This patch
allows to use the <feature> elements to either enable or disable
specific CPU flags. This can be used to force flags that can be emulated
even if the host CPU doesn't support them.

docs/formatdomain.html.in

@@ -901,11 +901,15 @@
           copying host CPU definition from capabilities XML into domain XML.
           Since the CPU definition is copied just before starting a domain,
           exactly the same XML can be used on different hosts while still
-          providing the best guest CPU each host supports. Neither
+          providing the best guest CPU each host supports. The
-          <code>match</code> attribute nor any <code>feature</code> elements
+          <code>match</code> attribute can't be used in this mode. Specifying
-          can be used in this mode. Specifying CPU model is not supported
+          CPU model is not supported either, but <code>model</code>'s
-          either, but <code>model</code>'s <code>fallback</code> attribute may
+          <code>fallback</code> attribute may still be used. Using the
-          still be used. Libvirt does not model every aspect of each CPU so
+          <code>feature</code> element, specific flags may be enabled or
+          disabled specifically in addition to the host model. This may be
+          used to fine tune features that can be emulated.
+          <span class="since">(Since 1.1.1)</span>.
+          Libvirt does not model every aspect of each CPU so
           the guest CPU will not match the host CPU exactly. On the other
           hand, the ABI provided to the guest is reproducible. During
           migration, complete CPU model definition is transferred to the

src/conf/cpu_conf.c

@@ -361,7 +361,7 @@ virCPUDefParseXML(const xmlNodePtr node,
         goto error;
 
     if (n > 0) {
-        if (!def->model) {
+        if (!def->model && def->mode != VIR_CPU_MODE_HOST_MODEL) {
             virReportError(VIR_ERR_XML_ERROR, "%s",
                            _("Non-empty feature list specified without "
                              "CPU model"));
@@ -574,7 +574,9 @@ virCPUDefFormatBuf(virBufferPtr buf,
                       (def->mode == VIR_CPU_MODE_HOST_MODEL ||
                        (def->mode == VIR_CPU_MODE_CUSTOM && def->model)));
 
-    if (!def->model && def->nfeatures) {
+    if (!def->model &&
+        def->mode != VIR_CPU_MODE_HOST_MODEL &&
+        def->nfeatures) {
         virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                        _("Non-empty feature list specified without CPU model"));
         return -1;
@@ -614,7 +616,6 @@ virCPUDefFormatBuf(virBufferPtr buf,
         virBufferAddLit(buf, "/>\n");
     }
 
-    if (formatModel) {
     for (i = 0; i < def->nfeatures; i++) {
         virCPUFeatureDefPtr feature = def->features + i;
 
@@ -641,7 +642,6 @@ virCPUDefFormatBuf(virBufferPtr buf,
                               feature->name);
         }
     }
-    }
 
     if (def->ncells) {
         virBufferAddLit(buf, "<numa>\n");

src/cpu/cpu_x86.c

@@ -1739,6 +1739,41 @@ cleanup:
     return ret;
 }
 
+
+static int
+x86UpdateHostModel(virCPUDefPtr guest,
+                   const virCPUDefPtr host)
+{
+    virCPUDefPtr oldguest;
+    size_t i;
+
+    guest->match = VIR_CPU_MATCH_EXACT;
+
+    /* no updates are required */
+    if (guest->nfeatures == 0) {
+        virCPUDefFreeModel(guest);
+        return virCPUDefCopyModel(guest, host, true);
+    }
+
+    /* update the host model according to the desired configuration */
+    if (!(oldguest = virCPUDefCopy(guest)))
+        return -1;
+
+    virCPUDefFreeModel(guest);
+    if (virCPUDefCopyModel(guest, host, true) < 0)
+        return -1;
+
+    for (i = 0; i < oldguest->nfeatures; i++) {
+        if (virCPUDefUpdateFeature(guest,
+                                   oldguest->features[i].name,
+                                   oldguest->features[i].policy) < 0)
+            return -1;
+    }
+
+    return 0;
+}
+
+
 static int
 x86Update(virCPUDefPtr guest,
           const virCPUDefPtr host)
@@ -1748,10 +1783,9 @@ x86Update(virCPUDefPtr guest,
         return x86UpdateCustom(guest, host);
 
     case VIR_CPU_MODE_HOST_MODEL:
+        return x86UpdateHostModel(guest, host);
+
     case VIR_CPU_MODE_HOST_PASSTHROUGH:
-        if (guest->mode == VIR_CPU_MODE_HOST_MODEL)
-            guest->match = VIR_CPU_MATCH_EXACT;
-        else
         guest->match = VIR_CPU_MATCH_MINIMUM;
         virCPUDefFreeModel(guest);
         return virCPUDefCopyModel(guest, host, true);