[PATCH v3] libxl: use qemu-xen with PV guests by default

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

qemu-xen offers better disk performances than qemu-xen-traditional
because it supports Linux native AIO: use it for PV guests if it is
available.

Changes in v3:
- use access instead of stat;
- log the errno.

Changes in v2:
- check for the existence of the qemu-xen binary before setting qemu-xen
as the default device model for PV guests.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
---
 tools/libxl/libxl_create.c |   33 ++++++++++++++++++++++++++++-----
 1 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index e63c7bd..f9c2a76 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -71,9 +71,34 @@ int libxl__domain_build_info_setdefault(libxl__gc *gc,
         b_info->type != LIBXL_DOMAIN_TYPE_PV)
         return ERROR_INVAL;
 
-    if (!b_info->device_model_version)
-        b_info->device_model_version =
-            LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN_TRADITIONAL;
+    libxl_defbool_setdefault(&b_info->device_model_stubdomain, false);
+
+    if (!b_info->device_model_version) {
+        if (b_info->type == LIBXL_DOMAIN_TYPE_HVM)
+            b_info->device_model_version =
+                LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN_TRADITIONAL;
+        else {
+            const char *dm;
+            int rc;
+
+            b_info->device_model_version =
+                LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN;
+            dm = libxl__domain_device_model(gc, b_info);
+            rc = access(dm, X_OK);
+            if (rc < 0) {
+                /* qemu-xen unavailable, use qemu-xen-traditional */
+                if (errno == ENOENT) {
+                    LIBXL__LOG_ERRNO(CTX, XTL_VERBOSE, "qemu-xen is unavailable"
+                            ", use qemu-xen-traditional instead");
+                    b_info->device_model_version =
+                        LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN_TRADITIONAL;
+                } else {
+                    LIBXL__LOG_ERRNO(CTX, XTL_ERROR, "qemu-xen access error");
+                    return ERROR_FAIL;
+                }
+            }
+        }
+    }
 
     if (b_info->type == LIBXL_DOMAIN_TYPE_HVM) {
         if (!b_info->u.hvm.bios)
@@ -99,8 +124,6 @@ int libxl__domain_build_info_setdefault(libxl__gc *gc,
         }
     }
 
-    libxl_defbool_setdefault(&b_info->device_model_stubdomain, false);
-
     if (b_info->type == LIBXL_DOMAIN_TYPE_HVM &&
         b_info->device_model_version !=
             LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN_TRADITIONAL &&
-- 
1.7.2.5