This is pretty simplistic for now, but I'd rather have someone better
friends with the tools improve it (if desired).

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -5958,6 +5958,26 @@ int libxl_send_debug_keys(libxl_ctx *ctx
     return 0;
 }
 
+int libxl_log_level(libxl_ctx *ctx, bool set, bool guest,
+                    int *lower_thresh, int *upper_thresh)
+{
+    int ret;
+    GC_INIT(ctx);
+    if (set) {
+        ret = xc_set_log_level(ctx->xch, guest, *lower_thresh, *upper_thresh);
+    } else {
+        ret = xc_get_log_level(ctx->xch, guest, lower_thresh, upper_thresh);
+    }
+    if ( ret < 0 ) {
+        LOGE(ERROR, "%s %slog level",
+             set ? "setting" : "getting", guest ? "guest " : "");
+        GC_FREE;
+        return ERROR_FAIL;
+    }
+    GC_FREE;
+    return 0;
+}
+
 libxl_xen_console_reader *
     libxl_xen_console_read_start(libxl_ctx *ctx, int clear)
 {
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -1765,6 +1765,8 @@ int libxl_send_trigger(libxl_ctx *ctx, u
                        libxl_trigger trigger, uint32_t vcpuid);
 int libxl_send_sysrq(libxl_ctx *ctx, uint32_t domid, char sysrq);
 int libxl_send_debug_keys(libxl_ctx *ctx, char *keys);
+int libxl_log_level(libxl_ctx *ctx, bool set, bool guest,
+                    int *lower_thresh, int *upper_thresh);
 
 typedef struct libxl__xen_console_reader libxl_xen_console_reader;
 
--- a/tools/libxl/xl.h
+++ b/tools/libxl/xl.h
@@ -81,6 +81,7 @@ int main_trigger(int argc, char **argv);
 int main_sysrq(int argc, char **argv);
 int main_debug_keys(int argc, char **argv);
 int main_dmesg(int argc, char **argv);
+int main_loglvl(int argc, char **argv);
 int main_top(int argc, char **argv);
 int main_networkattach(int argc, char **argv);
 int main_networklist(int argc, char **argv);
@@ -209,6 +210,8 @@ extern void printf_info_sexp(int domid,
 #define XL_GLOBAL_CONFIG XEN_CONFIG_DIR "/xl.conf"
 #define XL_LOCK_FILE XEN_LOCK_DIR "/xl"
 
+#define ARRAY_SIZE(a) (sizeof(a) / sizeof(a[0]))
+
 #endif /* XL_H */
 
 /*
--- a/tools/libxl/xl_cmdimpl.c
+++ b/tools/libxl/xl_cmdimpl.c
@@ -6469,6 +6469,84 @@ int main_debug_keys(int argc, char **arg
     return 0;
 }
 
+static const struct {
+    int level;
+    char string[8];
+} loglvls[] = {
+    { 0, "none" },
+    { 1, "error" },
+    { 2, "warning" },
+    { 3, "info" },
+    { 4, "all" },
+    { 4, "debug" },
+};
+
+static int parse_loglvl(char **parg)
+{
+    unsigned int i;
+
+    for (i = 0; i < ARRAY_SIZE(loglvls); ++i) {
+        size_t l = strlen(loglvls[i].string);
+
+        if (!strncmp(*parg, loglvls[i].string, l)) {
+            *parg += l;
+            return loglvls[i].level;
+        }
+    }
+
+    return -1;
+}
+
+static const char *format_loglvl(int loglvl)
+{
+    unsigned int i;
+
+    for (i = 0; i < ARRAY_SIZE(loglvls); ++i) {
+        if (loglvl == loglvls[i].level)
+            return loglvls[i].string;
+    }
+
+    return "<unknown>";
+}
+
+int main_loglvl(int argc, char **argv)
+{
+    static const struct option opts[] = {
+        {"guest", 0, 0, 'g'},
+        {"set", 0, 0, 's'},
+        COMMON_LONG_OPTS
+    };
+    int opt, lower_thresh = -1, upper_thresh = -1;
+    bool guest = false, set = false;
+
+    SWITCH_FOREACH_OPT(opt, "gs:", opts, "loglvl", 0) {
+    case 'g':
+        guest = true;
+        break;
+    case 's':
+        if (*optarg != '/')
+            lower_thresh = parse_loglvl(&optarg);
+        if (*optarg == '/') {
+            ++optarg;
+            upper_thresh = parse_loglvl(&optarg);
+        }
+        set = true;
+        break;
+    }
+
+    if (libxl_log_level(ctx, set, guest, &lower_thresh, &upper_thresh)) {
+        fprintf(stderr, "cannot %s %s log level\n",
+                set ? "set" : "get", guest ? "guest" : "host");
+        return 1;
+    }
+
+    if (!set)
+        printf("%s log levels: %s/%s\n", guest ? "guest" : "host",
+               format_loglvl(lower_thresh), format_loglvl(upper_thresh));
+
+    return 0;
+}
+
 int main_dmesg(int argc, char **argv)
 {
     unsigned int clear = 0;
--- a/tools/libxl/xl_cmdtable.c
+++ b/tools/libxl/xl_cmdtable.c
@@ -309,6 +309,13 @@ struct cmd_spec cmd_table[] = {
       "[-c]",
       "  -c                        Clear dmesg buffer as well as printing it",
     },
+    { "loglvl",
+      &main_loglvl, 0, 1,
+      "Manage Xen log levels",
+      "[-g] [-s=[LOWER][/UPPER]]",
+      "-g,                 --guest                 act on guest log level\n"
+      "-s [LOWER][/UPPER], --set=[LOWER][/UPPER]   set new log level\n"
+    },
     { "top",
       &main_top, 0, 0,
       "Monitor a host and the domains in real time",