From: Juergen Gross <jgross@suse.com>
Date: Wed, 30 Mar 2016 10:45:10 +0000 (+0200)
Subject: libxl: add query function for backend support by device model
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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@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
---

diff --git a/docs/misc/xenstore-paths.markdown b/docs/misc/xenstore-paths.markdown
index 087e239faa..7d5a06a47e 100644
--- a/docs/misc/xenstore-paths.markdown
+++ b/docs/misc/xenstore-paths.markdown
@@ -482,6 +482,13 @@ in the value supplied by the guest in this case).
 
 Contains the status of the device models running on the domain.
 
+#### ~/device-model/$DOMID/backends/* [w]
+
+Backend types the device model is supporting. Each entry below backends
+is a directory which may contain further nodes specific to the backend
+type. The name of each backend directory is the same as the backend type
+(e.g. "qdisk").
+
 #### ~/libxl/$DOMID/qdisk-backend-pid [w]
 
 Contains the PIDs of the device models running on the domain.
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index 29f6a37d1c..d2bb5ca8ef 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -2118,6 +2118,25 @@ 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 0896c0d85e..dbf0bd7a42 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -1619,6 +1619,11 @@ _hidden const char *libxl__domain_device_model(libxl__gc *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