3 files changed, 52 insertions, 12 deletions

diff --git a/docs/man/xl.conf.pod.5 b/docs/man/xl.conf.pod.5
index 7b9fcac2fa..a4ce3e529d 100644
--- a/docs/man/xl.conf.pod.5
+++ b/docs/man/xl.conf.pod.5
@@ -45,15 +45,22 @@ The semantics of each C<KEY> defines which form of C<VALUE> is required.
 
 =over 4
 
-=item B<autoballoon=BOOLEAN>
+=item B<autoballoon="off"|"on"|"auto">
 
-If disabled then C<xl> will not attempt to reduce the amount of memory
-assigned to domain 0 in order to create free memory when starting a
-new domain. You are strongly recommended to set this to C<0>
-(C<False>) if you use the C<dom0_mem> hypervisor command line to
-reduce the amount of memory given to domain 0 by default.
+If set to "on" then C<xl> will automatically reduce the amount of
+memory assigned to domain 0 in order to free memory for new domains.
 
-Default: C<1>
+If set to "off" then C<xl> will not automatically reduce the amount of
+domain 0 memory.
+
+If set to "auto" then auto-ballooning will be disabled if the
+C<dom0_mem> option was provided on the Xen command line.
+
+You are strongly recommended to set this to C<"off"> (or C<"auto">) if
+you use the C<dom0_mem> hypervisor command line to reduce the amount
+of memory given to domain 0 by default.
+
+Default: C<"on">
 
 =item B<run_hotplug_scripts=BOOLEAN>
 
diff --git a/tools/examples/xl.conf b/tools/examples/xl.conf
index b0caa3295b..50cba2bf37 100644
--- a/tools/examples/xl.conf
+++ b/tools/examples/xl.conf
@@ -1,8 +1,9 @@
 ## Global XL config file ##
 
-# automatically balloon down dom0 when xen doesn't have enough free
-# memory to create a domain
-#autoballoon=1
+# Control whether dom0 is ballooned down when xen doesn't have enough
+# free memory to create a domain.  "auto" means only balloon if dom0
+# starts with all the host's memory.
+#autoballoon="auto"
 
 # full path of the lockfile used by xl during domain creation
 #lockfile="/var/lock/xl"
diff --git a/tools/libxl/xl.c b/tools/libxl/xl.c
index 4c598dbbad..ac41fcd294 100644
--- a/tools/libxl/xl.c
+++ b/tools/libxl/xl.c
@@ -26,6 +26,7 @@
 #include <fcntl.h>
 #include <ctype.h>
 #include <inttypes.h>
+#include <regex.h>
 
 #include "libxl.h"
 #include "libxl_utils.h"
@@ -48,6 +49,29 @@ enum output_format default_output_format = OUTPUT_FORMAT_JSON;
 
 static xentoollog_level minmsglevel = XTL_PROGRESS;
 
+/* Get autoballoon option based on presence of dom0_mem Xen command
+   line option. */
+static int auto_autoballoon(void)
+{
+    const libxl_version_info *info;
+    regex_t regex;
+    int ret;
+
+    info = libxl_get_version_info(ctx);
+    if (!info)
+        return 1; /* default to on */
+
+    ret = regcomp(&regex,
+                  "(^| )dom0_mem=((|min:|max:)[0-9]+[bBkKmMgG]?,?)+($| )",
+                  REG_NOSUB | REG_EXTENDED);
+    if (ret)
+        return 1;
+
+    ret = regexec(&regex, info->commandline, 0, NULL, 0);
+    regfree(&regex);
+    return ret == REG_NOMATCH;
+}
+
 static void parse_global_config(const char *configfile,
                               const char *configfile_data,
                               int configfile_len)
@@ -69,8 +93,16 @@ static void parse_global_config(const char *configfile,
         exit(1);
     }
 
-    if (!xlu_cfg_get_long (config, "autoballoon", &l, 0))
-        autoballoon = l;
+    if (!xlu_cfg_get_string(config, "autoballoon", &buf, 0)) {
+        if (!strcmp(buf, "on") || !strcmp(buf, "1"))
+            autoballoon = 1;
+        else if (!strcmp(buf, "off") || !strcmp(buf, "0"))
+            autoballoon = 0;
+        else if (!strcmp(buf, "auto"))
+            autoballoon = auto_autoballoon();
+        else
+            fprintf(stderr, "invalid autoballoon option");
+    }
 
     if (!xlu_cfg_get_long (config, "run_hotplug_scripts", &l, 0))
         run_hotplug_scripts = l;