domain: cpu: Use host-passthrough by default on x86

When libvirt and qemu are new enough, use host-passthrough for the
CPU default. Nowadays this is recommended over host-model for most
end user usage where migration isn't a critical feature.

Signed-off-by: Cole Robinson <crobinso@redhat.com>

2 files changed, 33 insertions, 13 deletions

diff --git a/virtinst/domain/cpu.py b/virtinst/domain/cpu.py
index 23ded402..d74c3ffc 100644
--- a/virtinst/domain/cpu.py
+++ b/virtinst/domain/cpu.py
@@ -260,14 +260,26 @@ class DomainCpu(XMLBuilder):
                      SPECIAL_MODE_HOST_COPY, SPECIAL_MODE_HOST_MODEL,
                      SPECIAL_MODE_HOST_PASSTHROUGH, SPECIAL_MODE_CLEAR,
                      SPECIAL_MODE_APP_DEFAULT]
+
+    def _get_app_default_mode(self, guest):
+        # Depending on if libvirt+qemu is new enough, we prefer
+        # host-passthrough, then host-model, and finally host-model-only
+        domcaps = guest.lookup_domcaps()
+
+        if domcaps.supports_safe_host_passthrough():
+            return self.SPECIAL_MODE_HOST_PASSTHROUGH
+
+        log.debug("Safe host-passthrough is not available")
+        if domcaps.supports_safe_host_model():
+            return self.SPECIAL_MODE_HOST_MODEL
+
+        log.debug("Safe host-model is not available")
+        return self.SPECIAL_MODE_HOST_MODEL_ONLY
+
     def set_special_mode(self, guest, val):
         if val == self.SPECIAL_MODE_APP_DEFAULT:
-            # If libvirt is new enough to support reliable mode=host-model
-            # then use it, otherwise use previous default HOST_MODEL_ONLY
-            domcaps = guest.lookup_domcaps()
-            val = self.SPECIAL_MODE_HOST_MODEL_ONLY
-            if domcaps.supports_safe_host_model():
-                val = self.SPECIAL_MODE_HOST_MODEL
+            val = self._get_app_default_mode(guest)
+            log.debug("Using default cpu mode=%s", val)
 
         if (val == self.SPECIAL_MODE_HOST_MODEL or
             val == self.SPECIAL_MODE_HOST_PASSTHROUGH):
diff --git a/virtinst/domcapabilities.py b/virtinst/domcapabilities.py
index 4a217556..99a9a518 100644
--- a/virtinst/domcapabilities.py
+++ b/virtinst/domcapabilities.py
@@ -129,10 +129,9 @@ class _CPUModel(XMLBuilder):
     fallback = XMLProperty("./@fallback")
 
 
-class _CPUMode(XMLBuilder):
+class _CPUMode(_CapsBlock):
     XML_NAME = "mode"
     name = XMLProperty("./@name")
-    supported = XMLProperty("./@supported", is_yesno=True)
 
     models = XMLChildProperty(_CPUModel)
     def get_model(self, name):
@@ -265,11 +264,20 @@ class DomainCapabilities(XMLBuilder):
         host-model in fact predates this support, however it wasn't
         general purpose safe prior to domcaps advertisement.
         """
-        for m in self.cpu.modes:
-            if (m.name == "host-model" and m.supported and
-                    m.models[0].fallback == "forbid"):
-                return True
-        return False
+        m = self.cpu.get_mode("host-model")
+        return (m and m.supported and
+                m.models[0].fallback == "forbid")
+
+    def supports_safe_host_passthrough(self):
+        """
+        Return True if host-passthrough is safe enough to use by default.
+        We limit this to domcaps new enough to report whether host-passthrough
+        is migratable or not, which also means libvirt is about new enough
+        to not taint the VM for using host-passthrough
+        """
+        m = self.cpu.get_mode("host-passthrough")
+        return (m and m.supported and
+                "on" in m.get_enum("hostPassthroughMigratable").get_values())
 
     def get_cpu_models(self):
         models = []