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[Xen-devel] [PATCH v5 2/5] libxl: add query function for backend support by device model

Add a function to query whether the device model is supporting a
specific backend type. The device model is writing the supported
backend types to Xenstore on startup. The new query function checks
for the appropriate entry to be present.

As not all versions of qemu are capable to indicate support of
specific backends the query function is to be called with an indicator
whether the default return value should be "supported" (in case qemu
doesn't know set any support information) or "not supported".

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
 tools/libxl/libxl_dm.c       | 19 +++++++++++++++++++
 tools/libxl/libxl_internal.h |  5 +++++
 2 files changed, 24 insertions(+)

diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index 0d88c37..7d9abbe 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -1920,6 +1920,25 @@ out:
         device_model_spawn_outcome(egc, dmss, rc);
+bool libxl__query_qemu_backend(libxl__gc *gc, uint32_t domid,
+                               uint32_t backend_id, const char *type, bool def)
+    char *path;
+    char **dir;
+    unsigned int n;
+    path = GCSPRINTF("%s/device-model/%u/backends",
+                     libxl__xs_get_dompath(gc, backend_id), domid);
+    dir = libxl__xs_directory(gc, XBT_NULL, path, &n);
+    if (!dir)
+        return def;
+    path = GCSPRINTF("%s/device-model/%u/backends/%s",
+                     libxl__xs_get_dompath(gc, backend_id), domid, type);
+    dir = libxl__xs_directory(gc, XBT_NULL, path, &n);
+    return !!dir;
 static void device_model_confirm(libxl__egc *egc, libxl__spawn_state *spawn,
                                  const char *xsdata)
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index fc7bdab..c06ffc0 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -1617,6 +1617,11 @@ _hidden const char *libxl__domain_device_model(libxl__gc 
                                         const libxl_domain_build_info *info);
 _hidden int libxl__need_xenpv_qemu(libxl__gc *gc,
                                    libxl_domain_config *d_config);
+_hidden bool libxl__query_qemu_backend(libxl__gc *gc,
+                                       uint32_t domid,
+                                       uint32_t backend_id,
+                                       const char *type,
+                                       bool def);
  * This function will fix reserved device memory conflict

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