trace: Multi-backend tracing

Adds support to compile QEMU with multiple tracing backends at the same time.

For example, you can compile QEMU with:

  $ ./configure --enable-trace-backends=ftrace,dtrace

Where 'ftrace' can be handy for having an in-flight record of events, and 'dtrace' can be later used to extract more information from the system.

This patch allows having both available without recompiling QEMU.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

20 files changed, 153 insertions, 234 deletions

diff --git a/.travis.yml b/.travis.yml
index 04da9736..89c30ae 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -66,16 +66,16 @@ matrix:
       compiler: gcc
     # All the trace backends (apart from dtrace)
     - env: TARGETS=i386-softmmu,x86_64-softmmu
-           EXTRA_CONFIG="--enable-trace-backend=stderr"
+           EXTRA_CONFIG="--enable-trace-backends=stderr"
       compiler: gcc
     - env: TARGETS=i386-softmmu,x86_64-softmmu
-           EXTRA_CONFIG="--enable-trace-backend=simple"
+           EXTRA_CONFIG="--enable-trace-backends=simple"
       compiler: gcc
     - env: TARGETS=i386-softmmu,x86_64-softmmu
-           EXTRA_CONFIG="--enable-trace-backend=ftrace"
+           EXTRA_CONFIG="--enable-trace-backends=ftrace"
            TEST_CMD=""
       compiler: gcc
     - env: TARGETS=i386-softmmu,x86_64-softmmu
           EXTRA_PKGS="liblttng-ust-dev liburcu-dev"
-          EXTRA_CONFIG="--enable-trace-backend=ust"
+          EXTRA_CONFIG="--enable-trace-backends=ust"
       compiler: gcc
diff --git a/Makefile b/Makefile
index d830483..3c8f19c 100644
--- a/Makefile
+++ b/Makefile
@@ -52,12 +52,12 @@ GENERATED_HEADERS += trace/generated-events.h
 GENERATED_SOURCES += trace/generated-events.c
 
 GENERATED_HEADERS += trace/generated-tracers.h
-ifeq ($(TRACE_BACKEND),dtrace)
+ifeq ($(findstring dtrace,$(TRACE_BACKENDS)),dtrace)
 GENERATED_HEADERS += trace/generated-tracers-dtrace.h
 endif
 GENERATED_SOURCES += trace/generated-tracers.c
 
-ifeq ($(TRACE_BACKEND),ust)
+ifeq ($(findstring ust,$(TRACE_BACKENDS)),ust)
 GENERATED_HEADERS += trace/generated-ust-provider.h
 GENERATED_SOURCES += trace/generated-ust.c
 endif
diff --git a/Makefile.target b/Makefile.target
index 9986047..8155496 100644
--- a/Makefile.target
+++ b/Makefile.target
@@ -49,7 +49,7 @@ endif
 $(QEMU_PROG).stp-installed: $(SRC_PATH)/trace-events
 	$(call quiet-command,$(TRACETOOL) \
 		--format=stap \
-		--backend=$(TRACE_BACKEND) \
+		--backends=$(TRACE_BACKENDS) \
 		--binary=$(bindir)/$(QEMU_PROG) \
 		--target-name=$(TARGET_NAME) \
 		--target-type=$(TARGET_TYPE) \
@@ -58,7 +58,7 @@ $(QEMU_PROG).stp-installed: $(SRC_PATH)/trace-events
 $(QEMU_PROG).stp: $(SRC_PATH)/trace-events
 	$(call quiet-command,$(TRACETOOL) \
 		--format=stap \
-		--backend=$(TRACE_BACKEND) \
+		--backends=$(TRACE_BACKENDS) \
 		--binary=$(realpath .)/$(QEMU_PROG) \
 		--target-name=$(TARGET_NAME) \
 		--target-type=$(TARGET_TYPE) \
diff --git a/configure b/configure
index 0e516f9..a994f41 100755
--- a/configure
+++ b/configure
@@ -182,6 +182,10 @@ path_of() {
     return 1
 }
 
+have_backend () {
+    echo "$trace_backends" | grep "$1" >/dev/null
+}
+
 # default parameters
 source_path=`dirname "$0"`
 cpu=""
@@ -293,7 +297,7 @@ pkgversion=""
 pie=""
 zero_malloc=""
 qom_cast_debug="yes"
-trace_backend="nop"
+trace_backends="nop"
 trace_file="trace"
 spice=""
 rbd=""
@@ -753,7 +757,10 @@ for opt do
   ;;
   --target-list=*) target_list="$optarg"
   ;;
-  --enable-trace-backend=*) trace_backend="$optarg"
+  --enable-trace-backends=*) trace_backends="$optarg"
+  ;;
+  # XXX: backwards compatibility
+  --enable-trace-backend=*) trace_backends="$optarg"
   ;;
   --with-trace-file=*) trace_file="$optarg"
   ;;
@@ -1320,7 +1327,7 @@ Advanced options (experts only):
   --disable-docs           disable documentation build
   --disable-vhost-net      disable vhost-net acceleration support
   --enable-vhost-net       enable vhost-net acceleration support
-  --enable-trace-backend=B Set trace backend
+  --enable-trace-backends=B Set trace backend
                            Available backends: $($python $source_path/scripts/tracetool.py --list-backends)
   --with-trace-file=NAME   Full PATH,NAME of file to store traces
                            Default:trace-<pid>
@@ -3666,15 +3673,15 @@ fi
 ##########################################
 # check if trace backend exists
 
-$python "$source_path/scripts/tracetool.py" "--backend=$trace_backend" --check-backend  > /dev/null 2> /dev/null
+$python "$source_path/scripts/tracetool.py" "--backends=$trace_backends" --check-backends  > /dev/null 2> /dev/null
 if test "$?" -ne 0 ; then
-  error_exit "invalid trace backend" \
-      "Please choose a supported trace backend."
+  error_exit "invalid trace backends" \
+      "Please choose supported trace backends."
 fi
 
 ##########################################
 # For 'ust' backend, test if ust headers are present
-if test "$trace_backend" = "ust"; then
+if have_backend "ust"; then
   cat > $TMPC << EOF
 #include <lttng/tracepoint.h>
 int main(void) { return 0; }
@@ -3700,7 +3707,7 @@ fi
 
 ##########################################
 # For 'dtrace' backend, test if 'dtrace' command is present
-if test "$trace_backend" = "dtrace"; then
+if have_backend "dtrace"; then
   if ! has 'dtrace' ; then
     error_exit "dtrace command is not found in PATH $PATH"
   fi
@@ -4170,7 +4177,7 @@ echo "uuid support      $uuid"
 echo "libcap-ng support $cap_ng"
 echo "vhost-net support $vhost_net"
 echo "vhost-scsi support $vhost_scsi"
-echo "Trace backend     $trace_backend"
+echo "Trace backends    $trace_backends"
 if test "$trace_backend" = "simple"; then
 echo "Trace output file $trace_file-<pid>"
 fi
@@ -4664,43 +4671,35 @@ if test "$tpm" = "yes"; then
   fi
 fi
 
-# use default implementation for tracing backend-specific routines
-trace_default=yes
-echo "TRACE_BACKEND=$trace_backend" >> $config_host_mak
-if test "$trace_backend" = "nop"; then
+echo "TRACE_BACKENDS=$trace_backends" >> $config_host_mak
+if have_backend "nop"; then
   echo "CONFIG_TRACE_NOP=y" >> $config_host_mak
 fi
-if test "$trace_backend" = "simple"; then
+if have_backend "simple"; then
   echo "CONFIG_TRACE_SIMPLE=y" >> $config_host_mak
-  trace_default=no
   # Set the appropriate trace file.
   trace_file="\"$trace_file-\" FMT_pid"
 fi
-if test "$trace_backend" = "stderr"; then
+if have_backend "stderr"; then
   echo "CONFIG_TRACE_STDERR=y" >> $config_host_mak
-  trace_default=no
 fi
-if test "$trace_backend" = "ust"; then
+if have_backend "ust"; then
   echo "CONFIG_TRACE_UST=y" >> $config_host_mak
 fi
-if test "$trace_backend" = "dtrace"; then
+if have_backend "dtrace"; then
   echo "CONFIG_TRACE_DTRACE=y" >> $config_host_mak
   if test "$trace_backend_stap" = "yes" ; then
     echo "CONFIG_TRACE_SYSTEMTAP=y" >> $config_host_mak
   fi
 fi
-if test "$trace_backend" = "ftrace"; then
+if have_backend "ftrace"; then
   if test "$linux" = "yes" ; then
     echo "CONFIG_TRACE_FTRACE=y" >> $config_host_mak
-    trace_default=no
   else
     feature_not_found "ftrace(trace backend)" "ftrace requires Linux"
   fi
 fi
 echo "CONFIG_TRACE_FILE=$trace_file" >> $config_host_mak
-if test "$trace_default" = "yes"; then
-  echo "CONFIG_TRACE_DEFAULT=y" >> $config_host_mak
-fi
 
 if test "$rdma" = "yes" ; then
   echo "CONFIG_RDMA=y" >> $config_host_mak
diff --git a/docs/tracing.txt b/docs/tracing.txt
index bf2e15c..c6ab1c1 100644
--- a/docs/tracing.txt
+++ b/docs/tracing.txt
@@ -9,7 +9,7 @@ for debugging, profiling, and observing execution.
 
 1. Build with the 'simple' trace backend:
 
-    ./configure --enable-trace-backend=simple
+    ./configure --enable-trace-backends=simple
     make
 
 2. Create a file with the events you want to trace:
@@ -142,7 +142,7 @@ script.
 The trace backend is chosen at configure time and only one trace backend can
 be built into the binary:
 
-    ./configure --trace-backend=simple
+    ./configure --trace-backends=simple
 
 For a list of supported trace backends, try ./configure --help or see below.
 
diff --git a/qemu-io.c b/qemu-io.c
index 795cf46..b55a550 100644
--- a/qemu-io.c
+++ b/qemu-io.c
@@ -433,7 +433,7 @@ int main(int argc, char **argv)
             }
             break;
         case 'T':
-            if (!trace_backend_init(optarg, NULL)) {
+            if (!trace_init_backends(optarg, NULL)) {
                 exit(1); /* error message will have been printed */
             }
             break;
diff --git a/scripts/tracetool.py b/scripts/tracetool.py
index 5f4890f..83bde7b 100755
--- a/scripts/tracetool.py
+++ b/scripts/tracetool.py
@@ -6,7 +6,7 @@ Command-line wrapper for the tracetool machinery.
 """
 
 __author__     = "Lluís Vilanova <vilanova@ac.upc.edu>"
-__copyright__  = "Copyright 2012, Lluís Vilanova <vilanova@ac.upc.edu>"
+__copyright__  = "Copyright 2012-2014, Lluís Vilanova <vilanova@ac.upc.edu>"
 __license__    = "GPL version 2 or (at your option) any later version"
 
 __maintainer__ = "Stefan Hajnoczi"
@@ -32,7 +32,7 @@ def error_opt(msg = None):
     format_descr = "\n".join([ "    %-15s %s" % (n, d)
                                for n,d in tracetool.format.get_list() ])
     error_write("""\
-Usage: %(script)s --format=<format> --backend=<backend> [<options>]
+Usage: %(script)s --format=<format> --backends=<backends> [<options>]
 
 Backends:
 %(backends)s
@@ -43,7 +43,7 @@ Formats:
 Options:
     --help                   This help message.
     --list-backends          Print list of available backends.
-    --check-backend          Check if the given backend is valid.
+    --check-backends         Check if the given backend is valid.
     --binary <path>          Full path to QEMU binary.
     --target-type <type>     QEMU emulator target type ('system' or 'user').
     --target-name <name>     QEMU emulator target name.
@@ -65,16 +65,17 @@ def main(args):
     global _SCRIPT
     _SCRIPT = args[0]
 
-    long_opts  = [ "backend=", "format=", "help", "list-backends", "check-backend" ]
-    long_opts += [ "binary=", "target-type=", "target-name=", "probe-prefix=" ]
+    long_opts = ["backends=", "format=", "help", "list-backends",
+                 "check-backends"]
+    long_opts += ["binary=", "target-type=", "target-name=", "probe-prefix="]
 
     try:
         opts, args = getopt.getopt(args[1:], "", long_opts)
     except getopt.GetoptError, err:
         error_opt(str(err))
 
-    check_backend = False
-    arg_backend = ""
+    check_backends = False
+    arg_backends = []
     arg_format = ""
     binary = None
     target_type = None
@@ -84,8 +85,8 @@ def main(args):
         if opt == "--help":
             error_opt()
 
-        elif opt == "--backend":
-            arg_backend = arg
+        elif opt == "--backends":
+            arg_backends = arg.split(",")
         elif opt == "--format":
             arg_format = arg
 
@@ -93,8 +94,8 @@ def main(args):
             public_backends = tracetool.backend.get_list(only_public = True)
             out(", ".join([ b for b,_ in public_backends ]))
             sys.exit(0)
-        elif opt == "--check-backend":
-            check_backend = True
+        elif opt == "--check-backends":
+            check_backends = True
 
         elif opt == "--binary":
             binary = arg
@@ -108,14 +109,14 @@ def main(args):
         else:
             error_opt("unhandled option: %s" % opt)
 
-    if arg_backend is None:
-        error_opt("backend not set")
+    if len(arg_backends) == 0:
+        error_opt("no backends specified")
 
-    if check_backend:
-        if tracetool.backend.exists(arg_backend):
-            sys.exit(0)
-        else:
-            sys.exit(1)
+    if check_backends:
+        for backend in arg_backends:
+            if not tracetool.backend.exists(backend):
+                sys.exit(1)
+        sys.exit(0)
 
     if arg_format == "stap":
         if binary is None:
@@ -126,11 +127,11 @@ def main(args):
             error_opt("--target-name is required for SystemTAP tapset generator")
 
         if probe_prefix is None:
-            probe_prefix = ".".join([ "qemu", target_type, target_name ])
+            probe_prefix = ".".join(["qemu", target_type, target_name])
 
     try:
-        tracetool.generate(sys.stdin, arg_format, arg_backend,
-                           binary = binary, probe_prefix = probe_prefix)
+        tracetool.generate(sys.stdin, arg_format, arg_backends,
+                           binary=binary, probe_prefix=probe_prefix)
     except tracetool.TracetoolError, e:
         error_opt(str(e))
 
diff --git a/scripts/tracetool/__init__.py b/scripts/tracetool/__init__.py
index eccf552..e8e8edc 100644
--- a/scripts/tracetool/__init__.py
+++ b/scripts/tracetool/__init__.py
@@ -233,9 +233,9 @@ def try_import(mod_name, attr_name=None, attr_default=None):
         return False, None
 
 
-def generate(fevents, format, backend,
+def generate(fevents, format, backends,
              binary=None, probe_prefix=None):
-    """Generate the output for the given (format, backend) pair.
+    """Generate the output for the given (format, backends) pair.
 
     Parameters
     ----------
@@ -243,8 +243,8 @@ def generate(fevents, format, backend,
         Event description file.
     format : str
         Output format name.
-    backend : str
-        Output backend name.
+    backends : list
+        Output backend names.
     binary : str or None
         See tracetool.backend.dtrace.BINARY.
     probe_prefix : str or None
@@ -258,15 +258,13 @@ def generate(fevents, format, backend,
         raise TracetoolError("format not set")
     if not tracetool.format.exists(format):
         raise TracetoolError("unknown format: %s" % format)
-    format = format.replace("-", "_")
-
-    backend = str(backend)
-    if len(backend) is 0:
-        raise TracetoolError("backend not set")
-    if not tracetool.backend.exists(backend):
-        raise TracetoolError("unknown backend: %s" % backend)
-    backend = backend.replace("-", "_")
-    backend = tracetool.backend.Wrapper(backend, format)
+
+    if len(backends) is 0:
+        raise TracetoolError("no backends specified")
+    for backend in backends:
+        if not tracetool.backend.exists(backend):
+            raise TracetoolError("unknown backend: %s" % backend)
+    backend = tracetool.backend.Wrapper(backends, format)
 
     import tracetool.backend.dtrace
     tracetool.backend.dtrace.BINARY = binary
diff --git a/scripts/tracetool/backend/__init__.py b/scripts/tracetool/backend/__init__.py
index 5e36f04..5bfa1ef 100644
--- a/scripts/tracetool/backend/__init__.py
+++ b/scripts/tracetool/backend/__init__.py
@@ -99,17 +99,18 @@ def exists(name):
 
 
 class Wrapper:
-    def __init__(self, backend, format):
-        self._backend = backend.replace("-", "_")
+    def __init__(self, backends, format):
+        self._backends = [backend.replace("-", "_") for backend in backends]
         self._format = format.replace("-", "_")
-        assert exists(self._backend)
+        assert all(exists(backend) for backend in self._backends)
         assert tracetool.format.exists(self._format)
 
     def _run_function(self, name, *args, **kwargs):
-        func = tracetool.try_import("tracetool.backend." + self._backend,
-                                    name % self._format, None)[1]
-        if func is not None:
-            func(*args, **kwargs)
+        for backend in self._backends:
+            func = tracetool.try_import("tracetool.backend." + backend,
+                                        name % self._format, None)[1]
+            if func is not None:
+                func(*args, **kwargs)
 
     def generate_begin(self, events):
         self._run_function("generate_%s_begin", events)
diff --git a/trace/Makefile.objs b/trace/Makefile.objs
index 6a30467..d7a8696 100644
--- a/trace/Makefile.objs
+++ b/trace/Makefile.objs
@@ -3,12 +3,12 @@
 ######################################################################
 # Auto-generated event descriptions for LTTng ust code
 
-ifeq ($(TRACE_BACKEND),ust)
+ifeq ($(findstring ust,$(TRACE_BACKENDS)),ust)
 $(obj)/generated-ust-provider.h: $(obj)/generated-ust-provider.h-timestamp
 $(obj)/generated-ust-provider.h-timestamp: $(SRC_PATH)/trace-events
 	$(call quiet-command,$(TRACETOOL) \
 		--format=ust-events-h \
-		--backend=$(TRACE_BACKEND) \
+		--backends=$(TRACE_BACKENDS) \
 		< $< > $@,"  GEN   $(patsubst %-timestamp,%,$@)")
 	@cmp -s $@ $(patsubst %-timestamp,%,$@) || cp $@ $(patsubst %-timestamp,%,$@)
 
@@ -16,7 +16,7 @@ $(obj)/generated-ust.c: $(obj)/generated-ust.c-timestamp $(BUILD_DIR)/config-hos
 $(obj)/generated-ust.c-timestamp: $(SRC_PATH)/trace-events
 	$(call quiet-command,$(TRACETOOL) \
 		--format=ust-events-c \
-		--backend=$(TRACE_BACKEND) \
+		--backends=$(TRACE_BACKENDS) \
 		< $< > $@,"  GEN   $(patsubst %-timestamp,%,$@)")
 	@cmp -s $@ $(patsubst %-timestamp,%,$@) || cp $@ $(patsubst %-timestamp,%,$@)
 
@@ -31,7 +31,7 @@ $(obj)/generated-events.h: $(obj)/generated-events.h-timestamp
 $(obj)/generated-events.h-timestamp: $(SRC_PATH)/trace-events
 	$(call quiet-command,$(TRACETOOL) \
 		--format=events-h \
-		--backend=$(TRACE_BACKEND) \
+		--backends=$(TRACE_BACKENDS) \
 		< $< > $@,"  GEN   $(patsubst %-timestamp,%,$@)")
 	@cmp -s $@ $(patsubst %-timestamp,%,$@) || cp $@ $(patsubst %-timestamp,%,$@)
 
@@ -39,7 +39,7 @@ $(obj)/generated-events.c: $(obj)/generated-events.c-timestamp $(BUILD_DIR)/conf
 $(obj)/generated-events.c-timestamp: $(SRC_PATH)/trace-events
 	$(call quiet-command,$(TRACETOOL) \
 		--format=events-c \
-		--backend=$(TRACE_BACKEND) \
+		--backends=$(TRACE_BACKENDS) \
 		< $< > $@,"  GEN   $(patsubst %-timestamp,%,$@)")
 	@cmp -s $@ $(patsubst %-timestamp,%,$@) || cp $@ $(patsubst %-timestamp,%,$@)
 
@@ -54,23 +54,21 @@ $(obj)/generated-tracers.h: $(obj)/generated-tracers.h-timestamp
 $(obj)/generated-tracers.h-timestamp: $(SRC_PATH)/trace-events $(BUILD_DIR)/config-host.mak
 	$(call quiet-command,$(TRACETOOL) \
 		--format=h \
-		--backend=$(TRACE_BACKEND) \
+		--backends=$(TRACE_BACKENDS) \
 		< $< > $@,"  GEN   $(patsubst %-timestamp,%,$@)")
 
 ######################################################################
 # Auto-generated tracing routines (non-DTrace)
 
-ifneq ($(TRACE_BACKEND),dtrace)
 $(obj)/generated-tracers.c: $(obj)/generated-tracers.c-timestamp
 	@cmp -s $< $@ || cp $< $@
 $(obj)/generated-tracers.c-timestamp: $(SRC_PATH)/trace-events $(BUILD_DIR)/config-host.mak
 	$(call quiet-command,$(TRACETOOL) \
 		--format=c \
-		--backend=$(TRACE_BACKEND) \
+		--backends=$(TRACE_BACKENDS) \
 		< $< > $@,"  GEN   $(patsubst %-timestamp,%,$@)")
 
 $(obj)/generated-tracers.o: $(obj)/generated-tracers.c $(obj)/generated-tracers.h
-endif
 
 
 ######################################################################
@@ -79,27 +77,27 @@ endif
 # Normal practice is to name DTrace probe file with a '.d' extension
 # but that gets picked up by QEMU's Makefile as an external dependency
 # rule file. So we use '.dtrace' instead
-ifeq ($(TRACE_BACKEND),dtrace)
-$(obj)/generated-tracers.dtrace: $(obj)/generated-tracers.dtrace-timestamp
-$(obj)/generated-tracers.dtrace-timestamp: $(SRC_PATH)/trace-events $(BUILD_DIR)/config-host.mak
+ifeq ($(findstring dtrace,$(TRACE_BACKENDS)),dtrace)
+$(obj)/generated-tracers-dtrace.dtrace: $(obj)/generated-tracers-dtrace.dtrace-timestamp
+$(obj)/generated-tracers-dtrace.dtrace-timestamp: $(SRC_PATH)/trace-events $(BUILD_DIR)/config-host.mak
 	$(call quiet-command,$(TRACETOOL) \
 		--format=d \
-		--backend=$(TRACE_BACKEND) \
+		--backends=$(TRACE_BACKENDS) \
 		< $< > $@,"  GEN   $(patsubst %-timestamp,%,$@)")
 	@cmp -s $@ $(patsubst %-timestamp,%,$@) || cp $@ $(patsubst %-timestamp,%,$@)
 
-$(obj)/generated-tracers-dtrace.h: $(obj)/generated-tracers.dtrace
+$(obj)/generated-tracers-dtrace.h: $(obj)/generated-tracers-dtrace.dtrace
 	$(call quiet-command,dtrace -o $@ -h -s $<, "  GEN   $@")
 
-$(obj)/generated-tracers.o: $(obj)/generated-tracers.dtrace
+$(obj)/generated-tracers-dtrace.o: $(obj)/generated-tracers-dtrace.dtrace
+
+util-obj-y += generated-tracers-dtrace.o
 endif
 
 ######################################################################
 # Backend code
 
-util-obj-$(CONFIG_TRACE_DEFAULT) += default.o
 util-obj-$(CONFIG_TRACE_SIMPLE) += simple.o
-util-obj-$(CONFIG_TRACE_STDERR) += stderr.o
 util-obj-$(CONFIG_TRACE_FTRACE) += ftrace.o
 util-obj-$(CONFIG_TRACE_UST) += generated-ust.o
 util-obj-y += control.o
diff --git a/trace/control-internal.h b/trace/control-internal.h
index b3f587e..5a8df28 100644
--- a/trace/control-internal.h
+++ b/trace/control-internal.h
@@ -1,7 +1,7 @@
 /*
  * Interface for configuring and controlling the state of tracing events.
  *
- * Copyright (C) 2011-2012 Lluís Vilanova <vilanova@ac.upc.edu>
+ * Copyright (C) 2011-2014 Lluís Vilanova <vilanova@ac.upc.edu>
  *
  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  * See the COPYING file in the top-level directory.
@@ -61,7 +61,7 @@ static inline void trace_event_set_state_dynamic(TraceEvent *ev, bool state)
 {
     assert(ev != NULL);
     assert(trace_event_get_state_static(ev));
-    return trace_event_set_state_dynamic_backend(ev, state);
+    ev->dstate = state;
 }
 
 #endif  /* TRACE__CONTROL_INTERNAL_H */
diff --git a/trace/control.c b/trace/control.c
index 4aa02cf..45e6604 100644
--- a/trace/control.c
+++ b/trace/control.c
@@ -1,13 +1,19 @@
 /*
  * Interface for configuring and controlling the state of tracing events.
  *
- * Copyright (C) 2011-2012 Lluís Vilanova <vilanova@ac.upc.edu>
+ * Copyright (C) 2011-2014 Lluís Vilanova <vilanova@ac.upc.edu>
  *
  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  * See the COPYING file in the top-level directory.
  */
 
 #include "trace/control.h"
+#ifdef CONFIG_TRACE_SIMPLE
+#include "trace/simple.h"
+#endif
+#ifdef CONFIG_TRACE_FTRACE
+#include "trace/ftrace.h"
+#endif
 
 
 TraceEvent *trace_event_name(const char *name)
@@ -79,7 +85,20 @@ TraceEvent *trace_event_pattern(const char *pat, TraceEvent *ev)
     return NULL;
 }
 
-void trace_backend_init_events(const char *fname)
+void trace_print_events(FILE *stream, fprintf_function stream_printf)
+{
+    TraceEventID i;
+
+    for (i = 0; i < trace_event_count(); i++) {
+        TraceEvent *ev = trace_event_id(i);
+        stream_printf(stream, "%s [Event ID %u] : state %u\n",
+                      trace_event_get_name(ev), i,
+                      trace_event_get_state_static(ev) &&
+                      trace_event_get_state_dynamic(ev));
+    }
+}
+
+static void trace_init_events(const char *fname)
 {
     if (fname == NULL) {
         return;
@@ -130,3 +149,29 @@ void trace_backend_init_events(const char *fname)
         exit(1);
     }
 }
+
+bool trace_init_backends(const char *events, const char *file)
+{
+#ifdef CONFIG_TRACE_SIMPLE
+    if (!st_init(file)) {
+        fprintf(stderr, "failed to initialize simple tracing backend.\n");
+        return false;
+    }
+#else
+    if (file) {
+        fprintf(stderr, "error: -trace file=...: "
+                "option not supported by the selected tracing backends\n");
+        return false;
+    }
+#endif
+
+#ifdef CONFIG_TRACE_FTRACE
+    if (!ftrace_init()) {
+        fprintf(stderr, "failed to initialize ftrace backend.\n");
+        return false;
+    }
+#endif
+
+    trace_init_events(events);
+    return true;
+}
diff --git a/trace/control.h b/trace/control.h
index cde8260..e1ec033 100644
--- a/trace/control.h
+++ b/trace/control.h
@@ -1,7 +1,7 @@
 /*
  * Interface for configuring and controlling the state of tracing events.
  *
- * Copyright (C) 2011-2012 Lluís Vilanova <vilanova@ac.upc.edu>
+ * Copyright (C) 2011-2014 Lluís Vilanova <vilanova@ac.upc.edu>
  *
  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  * See the COPYING file in the top-level directory.
@@ -146,26 +146,17 @@ static bool trace_event_get_state_dynamic(TraceEvent *ev);
  */
 static void trace_event_set_state_dynamic(TraceEvent *ev, bool state);
 
-/**
- * trace_event_set_state_dynamic_backend:
- *
- * Warning: This function must be implemented by each tracing backend.
- */
-void trace_event_set_state_dynamic_backend(TraceEvent *ev, bool state);
-
 
 
 /**
  * trace_print_events:
  *
  * Print the state of all events.
- *
- * Warning: This function must be implemented by each tracing backend.
  */
 void trace_print_events(FILE *stream, fprintf_function stream_printf);
 
 /**
- * trace_backend_init:
+ * trace_init_backends:
  * @events: Name of file with events to be enabled at startup; may be NULL.
  *          Corresponds to commandline option "-trace events=...".
  * @file:   Name of trace output file; may be NULL.
@@ -173,19 +164,9 @@ void trace_print_events(FILE *stream, fprintf_function stream_printf);
  *
  * Initialize the tracing backend.
  *
- * Warning: This function must be implemented by each tracing backend.
- *
- * Returns: Whether the backend could be successfully initialized.
- */
-bool trace_backend_init(const char *events, const char *file);
-
-/**
- * trace_backend_init_events:
- * @fname: Name of file with events to enable; may be NULL.
- *
- * Generic function to initialize the state of events.
+ * Returns: Whether the backends could be successfully initialized.
  */
-void trace_backend_init_events(const char *fname);
+bool trace_init_backends(const char *events, const char *file);
 
 
 #include "trace/control-internal.h"
diff --git a/trace/default.c b/trace/default.c
deleted file mode 100644
index 6e07a47..0000000
--- a/trace/default.c
+++ /dev/null
@@ -1,40 +0,0 @@
-/*
- * Default implementation for backend initialization from commandline.
- *
- * Copyright (C) 2011-2012 Lluís Vilanova <vilanova@ac.upc.edu>
- *
- * This work is licensed under the terms of the GNU GPL, version 2.  See
- * the COPYING file in the top-level directory.
- */
-
-#include "trace/control.h"
-
-
-void trace_print_events(FILE *stream, fprintf_function stream_printf)
-{
-    fprintf(stderr, "warning: "
-            "cannot print the trace events with the current backend\n");
-    stream_printf(stream, "error: "
-                  "operation not supported with the current backend\n");
-}
-
-void trace_event_set_state_dynamic_backend(TraceEvent *ev, bool state)
-{
-    fprintf(stderr, "warning: "
-            "cannot set the state of a trace event with the current backend\n");
-}
-
-bool trace_backend_init(const char *events, const char *file)
-{
-    if (events) {
-        fprintf(stderr, "error: -trace events=...: "
-                "option not supported by the selected tracing backend\n");
-        return false;
-    }
-    if (file) {
-        fprintf(stderr, "error: -trace file=...: "
-                "option not supported by the selected tracing backend\n");
-        return false;
-    }
-    return true;
-}
diff --git a/trace/ftrace.c b/trace/ftrace.c
index 46b7fdb..a7ae371 100644
--- a/trace/ftrace.c
+++ b/trace/ftrace.c
@@ -42,35 +42,13 @@ static int find_debugfs(char *debugfs)
     return 1;
 }
 
-void trace_print_events(FILE *stream, fprintf_function stream_printf)
-{
-    TraceEventID i;
-
-    for (i = 0; i < trace_event_count(); i++) {
-        TraceEvent *ev = trace_event_id(i);
-        stream_printf(stream, "%s [Event ID %u] : state %u\n",
-                      trace_event_get_name(ev), i, trace_event_get_state_dynamic(ev));
-    }
-}
-
-void trace_event_set_state_dynamic_backend(TraceEvent *ev, bool state)
-{
-    ev->dstate = state;
-}
-
-bool trace_backend_init(const char *events, const char *file)
+bool ftrace_init(void)
 {
     char debugfs[PATH_MAX];
     char path[PATH_MAX];
     int debugfs_found;
     int trace_fd = -1;
 
-    if (file) {
-        fprintf(stderr, "error: -trace file=...: "
-                "option not supported by the selected tracing backend\n");
-        return false;
-    }
-
     debugfs_found = find_debugfs(debugfs);
     if (debugfs_found) {
         snprintf(path, PATH_MAX, "%s/tracing/tracing_on", debugfs);
@@ -97,6 +75,5 @@ bool trace_backend_init(const char *events, const char *file)
         return false;
     }
 
-    trace_backend_init_events(events);
     return true;
 }
diff --git a/trace/ftrace.h b/trace/ftrace.h
index 94cb8d5..863e052 100644
--- a/trace/ftrace.h
+++ b/trace/ftrace.h
@@ -1,10 +1,15 @@
 #ifndef TRACE_FTRACE_H
 #define TRACE_FTRACE_H
 
+#include <stdbool.h>
+
+
 #define MAX_TRACE_STRLEN 512
 #define _STR(x) #x
 #define STR(x) _STR(x)
 
 extern int trace_marker_fd;
 
+bool ftrace_init(void);
+
 #endif /* ! TRACE_FTRACE_H */
diff --git a/trace/simple.c b/trace/simple.c
index 5a2e188..a8f923c 100644
--- a/trace/simple.c
+++ b/trace/simple.c
@@ -368,22 +368,6 @@ void st_flush_trace_buffer(void)
     flush_trace_file(true);
 }
 
-void trace_print_events(FILE *stream, fprintf_function stream_printf)
-{
-    unsigned int i;
-
-    for (i = 0; i < trace_event_count(); i++) {
-        TraceEvent *ev = trace_event_id(i);
-        stream_printf(stream, "%s [Event ID %u] : state %u\n",
-                      trace_event_get_name(ev), i, trace_event_get_state_dynamic(ev));
-    }
-}
-
-void trace_event_set_state_dynamic_backend(TraceEvent *ev, bool state)
-{
-    ev->dstate = state;
-}
-
 /* Helper function to create a thread with signals blocked.  Use glib's
  * portable threads since QEMU abstractions cannot be used due to reentrancy in
  * the tracer.  Also note the signal masking on POSIX hosts so that the thread
@@ -412,7 +396,7 @@ static GThread *trace_thread_create(GThreadFunc fn)
     return thread;
 }
 
-bool trace_backend_init(const char *events, const char *file)
+bool st_init(const char *file)
 {
     GThread *thread;
 
@@ -430,7 +414,6 @@ bool trace_backend_init(const char *events, const char *file)
     }
 
     atexit(st_flush_trace_buffer);
-    trace_backend_init_events(events);
     st_set_trace_file(file);
     return true;
 }
diff --git a/trace/simple.h b/trace/simple.h
index 5260d9a..6997996 100644
--- a/trace/simple.h
+++ b/trace/simple.h
@@ -21,6 +21,7 @@
 void st_print_trace_file_status(FILE *stream, fprintf_function stream_printf);
 void st_set_trace_file_enabled(bool enable);
 bool st_set_trace_file(const char *file);
+bool st_init(const char *file);
 void st_flush_trace_buffer(void);
 
 typedef struct {
diff --git a/trace/stderr.c b/trace/stderr.c
deleted file mode 100644
index e212efd..0000000
--- a/trace/stderr.c
+++ /dev/null
@@ -1,30 +0,0 @@
-#include "trace.h"
-#include "trace/control.h"
-
-
-void trace_print_events(FILE *stream, fprintf_function stream_printf)
-{
-    TraceEventID i;
-
-    for (i = 0; i < trace_event_count(); i++) {
-        TraceEvent *ev = trace_event_id(i);
-        stream_printf(stream, "%s [Event ID %u] : state %u\n",
-                      trace_event_get_name(ev), i, trace_event_get_state_dynamic(ev));
-    }
-}
-
-void trace_event_set_state_dynamic_backend(TraceEvent *ev, bool state)
-{
-    ev->dstate = state;
-}
-
-bool trace_backend_init(const char *events, const char *file)
-{
-    if (file) {
-        fprintf(stderr, "error: -trace file=...: "
-                "option not supported by the selected tracing backend\n");
-        return false;
-    }
-    trace_backend_init_events(events);
-    return true;
-}
diff --git a/vl.c b/vl.c
index a3def97..ac0e3d7 100644
--- a/vl.c
+++ b/vl.c
@@ -4039,7 +4039,7 @@ int main(int argc, char **argv, char **envp)
     }
 
     if (!is_daemonized()) {
-        if (!trace_backend_init(trace_events, trace_file)) {
+        if (!trace_init_backends(trace_events, trace_file)) {
             exit(1);
         }
     }
@@ -4559,7 +4559,7 @@ int main(int argc, char **argv, char **envp)
     os_setup_post();
 
     if (is_daemonized()) {
-        if (!trace_backend_init(trace_events, trace_file)) {
+        if (!trace_init_backends(trace_events, trace_file)) {
             exit(1);
         }
     }