trace/simple.c - third_party/qemu - Git at Google

 /*
  * Simple trace backend
  *
  * Copyright IBM, Corp. 2010
  *
  * This work is licensed under the terms of the GNU GPL, version 2.  See
  * the COPYING file in the top-level directory.
  *
  */

 #include "qemu/osdep.h"
 #ifndef _WIN32
 #include <pthread.h>
 #endif
 #include "qemu/timer.h"
 #include "trace/control.h"
 #include "trace/simple.h"
 #include "qemu/error-report.h"
 #include "qemu/qemu-print.h"

 /** Trace file header event ID, picked to avoid conflict with real event IDs */
 #define HEADER_EVENT_ID (~(uint64_t)0)

 /** Trace file magic number */
 #define HEADER_MAGIC 0xf2b177cb0aa429b4ULL

 /** Trace file version number, bump if format changes */
 #define HEADER_VERSION 4

 /** Records were dropped event ID */
 #define DROPPED_EVENT_ID (~(uint64_t)0 - 1)

 /** Trace record is valid */
 #define TRACE_RECORD_VALID ((uint64_t)1 << 63)

 /*
  * Trace records are written out by a dedicated thread.  The thread waits for
  * records to become available, writes them out, and then waits again.
  */
 static GMutex trace_lock;
 static GCond trace_available_cond;
 static GCond trace_empty_cond;

 static bool trace_available;
 static bool trace_writeout_enabled;

 enum {
     TRACE_BUF_LEN = 4096 * 64,
     TRACE_BUF_FLUSH_THRESHOLD = TRACE_BUF_LEN / 4,
 };

 uint8_t trace_buf[TRACE_BUF_LEN];
 static volatile gint trace_idx;
 static unsigned int writeout_idx;
 static volatile gint dropped_events;
 static uint32_t trace_pid;
 static FILE *trace_fp;
 static char *trace_file_name;

 #define TRACE_RECORD_TYPE_MAPPING 0
 #define TRACE_RECORD_TYPE_EVENT   1

 /* * Trace buffer entry */
 typedef struct {
     uint64_t event; /* event ID value */
     uint64_t timestamp_ns;
     uint32_t length;   /*    in bytes */
     uint32_t pid;
     uint64_t arguments[];
 } TraceRecord;

 typedef struct {
     uint64_t header_event_id; /* HEADER_EVENT_ID */
     uint64_t header_magic;    /* HEADER_MAGIC    */
     uint64_t header_version;  /* HEADER_VERSION  */
 } TraceLogHeader;


 static void read_from_buffer(unsigned int idx, void *dataptr, size_t size);
 static unsigned int write_to_buffer(unsigned int idx, void *dataptr, size_t size);

 static void clear_buffer_range(unsigned int idx, size_t len)
 {
     uint32_t num = 0;
     while (num < len) {
         if (idx >= TRACE_BUF_LEN) {
             idx = idx % TRACE_BUF_LEN;
         }
         trace_buf[idx++] = 0;
         num++;
     }
 }
 /**
  * Read a trace record from the trace buffer
  *
  * @idx         Trace buffer index
  * @record      Trace record to fill
  *
  * Returns false if the record is not valid.
  */
 static bool get_trace_record(unsigned int idx, TraceRecord **recordptr)
 {
     uint64_t event_flag = 0;
     TraceRecord record;
     /* read the event flag to see if its a valid record */
     read_from_buffer(idx, &record, sizeof(event_flag));

     if (!(record.event & TRACE_RECORD_VALID)) {
         return false;
     }

     smp_rmb(); /* read memory barrier before accessing record */
     /* read the record header to know record length */
     read_from_buffer(idx, &record, sizeof(TraceRecord));
     *recordptr = malloc(record.length); /* don't use g_malloc, can deadlock when traced */
     /* make a copy of record to avoid being overwritten */
     read_from_buffer(idx, *recordptr, record.length);
     smp_rmb(); /* memory barrier before clearing valid flag */
     (*recordptr)->event &= ~TRACE_RECORD_VALID;
     /* clear the trace buffer range for consumed record otherwise any byte
      * with its MSB set may be considered as a valid event id when the writer
      * thread crosses this range of buffer again.
      */
     clear_buffer_range(idx, record.length);
     return true;
 }

 /**
  * Kick writeout thread
  *
  * @wait        Whether to wait for writeout thread to complete
  */
 static void flush_trace_file(bool wait)
 {
     g_mutex_lock(&trace_lock);
     trace_available = true;
     g_cond_signal(&trace_available_cond);

     if (wait) {
         g_cond_wait(&trace_empty_cond, &trace_lock);
     }

     g_mutex_unlock(&trace_lock);
 }

 static void wait_for_trace_records_available(void)
 {
     g_mutex_lock(&trace_lock);
     while (!(trace_available && trace_writeout_enabled)) {
         g_cond_signal(&trace_empty_cond);
         g_cond_wait(&trace_available_cond, &trace_lock);
     }
     trace_available = false;
     g_mutex_unlock(&trace_lock);
 }

 static gpointer writeout_thread(gpointer opaque)
 {
     TraceRecord *recordptr;
     union {
         TraceRecord rec;
         uint8_t bytes[sizeof(TraceRecord) + sizeof(uint64_t)];
     } dropped;
     unsigned int idx = 0;
     int dropped_count;
     size_t unused __attribute__ ((unused));
     uint64_t type = TRACE_RECORD_TYPE_EVENT;

     for (;;) {
         wait_for_trace_records_available();

         if (g_atomic_int_get(&dropped_events)) {
             dropped.rec.event = DROPPED_EVENT_ID;
             dropped.rec.timestamp_ns = get_clock();
             dropped.rec.length = sizeof(TraceRecord) + sizeof(uint64_t);
             dropped.rec.pid = trace_pid;
             do {
                 dropped_count = g_atomic_int_get(&dropped_events);
             } while (!g_atomic_int_compare_and_exchange(&dropped_events,
                                                         dropped_count, 0));
             dropped.rec.arguments[0] = dropped_count;
             unused = fwrite(&type, sizeof(type), 1, trace_fp);
             unused = fwrite(&dropped.rec, dropped.rec.length, 1, trace_fp);
         }

         while (get_trace_record(idx, &recordptr)) {
             unused = fwrite(&type, sizeof(type), 1, trace_fp);
             unused = fwrite(recordptr, recordptr->length, 1, trace_fp);
             writeout_idx += recordptr->length;
             free(recordptr); /* don't use g_free, can deadlock when traced */
             idx = writeout_idx % TRACE_BUF_LEN;
         }

         fflush(trace_fp);
     }
     return NULL;
 }

 void trace_record_write_u64(TraceBufferRecord *rec, uint64_t val)
 {
     rec->rec_off = write_to_buffer(rec->rec_off, &val, sizeof(uint64_t));
 }

 void trace_record_write_str(TraceBufferRecord *rec, const char *s, uint32_t slen)
 {
     /* Write string length first */
     rec->rec_off = write_to_buffer(rec->rec_off, &slen, sizeof(slen));
     /* Write actual string now */
     rec->rec_off = write_to_buffer(rec->rec_off, (void*)s, slen);
 }

 int trace_record_start(TraceBufferRecord *rec, uint32_t event, size_t datasize)
 {
     unsigned int idx, rec_off, old_idx, new_idx;
     uint32_t rec_len = sizeof(TraceRecord) + datasize;
     uint64_t event_u64 = event;
     uint64_t timestamp_ns = get_clock();

     do {
         old_idx = g_atomic_int_get(&trace_idx);
         smp_rmb();
         new_idx = old_idx + rec_len;

         if (new_idx - writeout_idx > TRACE_BUF_LEN) {
             /* Trace Buffer Full, Event dropped ! */
             g_atomic_int_inc(&dropped_events);
             return -ENOSPC;
         }
     } while (!g_atomic_int_compare_and_exchange(&trace_idx, old_idx, new_idx));

     idx = old_idx % TRACE_BUF_LEN;

     rec_off = idx;
     rec_off = write_to_buffer(rec_off, &event_u64, sizeof(event_u64));
     rec_off = write_to_buffer(rec_off, &timestamp_ns, sizeof(timestamp_ns));
     rec_off = write_to_buffer(rec_off, &rec_len, sizeof(rec_len));
     rec_off = write_to_buffer(rec_off, &trace_pid, sizeof(trace_pid));

     rec->tbuf_idx = idx;
     rec->rec_off  = (idx + sizeof(TraceRecord)) % TRACE_BUF_LEN;
     return 0;
 }

 static void read_from_buffer(unsigned int idx, void *dataptr, size_t size)
 {
     uint8_t *data_ptr = dataptr;
     uint32_t x = 0;
     while (x < size) {
         if (idx >= TRACE_BUF_LEN) {
             idx = idx % TRACE_BUF_LEN;
         }
         data_ptr[x++] = trace_buf[idx++];
     }
 }

 static unsigned int write_to_buffer(unsigned int idx, void *dataptr, size_t size)
 {
     uint8_t *data_ptr = dataptr;
     uint32_t x = 0;
     while (x < size) {
         if (idx >= TRACE_BUF_LEN) {
             idx = idx % TRACE_BUF_LEN;
         }
         trace_buf[idx++] = data_ptr[x++];
     }
     return idx; /* most callers wants to know where to write next */
 }

 void trace_record_finish(TraceBufferRecord *rec)
 {
     TraceRecord record;
     read_from_buffer(rec->tbuf_idx, &record, sizeof(TraceRecord));
     smp_wmb(); /* write barrier before marking as valid */
     record.event |= TRACE_RECORD_VALID;
     write_to_buffer(rec->tbuf_idx, &record, sizeof(TraceRecord));

     if (((unsigned int)g_atomic_int_get(&trace_idx) - writeout_idx)
         > TRACE_BUF_FLUSH_THRESHOLD) {
         flush_trace_file(false);
     }
 }

 static int st_write_event_mapping(TraceEventIter *iter)
 {
     uint64_t type = TRACE_RECORD_TYPE_MAPPING;
     TraceEvent *ev;

     while ((ev = trace_event_iter_next(iter)) != NULL) {
         uint64_t id = trace_event_get_id(ev);
         const char *name = trace_event_get_name(ev);
         uint32_t len = strlen(name);
         if (fwrite(&type, sizeof(type), 1, trace_fp) != 1 ||
             fwrite(&id, sizeof(id), 1, trace_fp) != 1 ||
             fwrite(&len, sizeof(len), 1, trace_fp) != 1 ||
             fwrite(name, len, 1, trace_fp) != 1) {
             return -1;
         }
     }

     return 0;
 }

 /**
  * Enable / disable tracing, return whether it was enabled.
  *
  * @enable: enable if %true, else disable.
  */
 bool st_set_trace_file_enabled(bool enable)
 {
     TraceEventIter iter;
     bool was_enabled = trace_fp;

     if (enable == !!trace_fp) {
         return was_enabled;     /* no change */
     }

     /* Halt trace writeout */
     flush_trace_file(true);
     trace_writeout_enabled = false;
     flush_trace_file(true);

     if (enable) {
         static const TraceLogHeader header = {
             .header_event_id = HEADER_EVENT_ID,
             .header_magic = HEADER_MAGIC,
             /* Older log readers will check for version at next location */
             .header_version = HEADER_VERSION,
         };

         trace_fp = fopen(trace_file_name, "wb");
         if (!trace_fp) {
             return was_enabled;
         }

         trace_event_iter_init_all(&iter);
         if (fwrite(&header, sizeof header, 1, trace_fp) != 1 ||
             st_write_event_mapping(&iter) < 0) {
             fclose(trace_fp);
             trace_fp = NULL;
             return was_enabled;
         }

         /* Resume trace writeout */
         trace_writeout_enabled = true;
         flush_trace_file(false);
     } else {
         fclose(trace_fp);
         trace_fp = NULL;
     }
     return was_enabled;
 }

 /**
  * Set the name of a trace file
  *
  * @file        The trace file name or NULL for the default name-<pid> set at
  *              config time
  */
 void st_set_trace_file(const char *file)
 {
     bool saved_enable = st_set_trace_file_enabled(false);

     g_free(trace_file_name);

     if (!file) {
         /* Type cast needed for Windows where getpid() returns an int. */
         trace_file_name = g_strdup_printf(CONFIG_TRACE_FILE "-" FMT_pid, (pid_t)getpid());
     } else {
         trace_file_name = g_strdup_printf("%s", file);
     }

     st_set_trace_file_enabled(saved_enable);
 }

 void st_print_trace_file_status(void)
 {
     qemu_printf("Trace file \"%s\" %s.\n",
                 trace_file_name, trace_fp ? "on" : "off");
 }

 void st_flush_trace_buffer(void)
 {
     flush_trace_file(true);
 }

 /* 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
  * does not steal signals when the rest of the program wants them blocked.
  */
 static GThread *trace_thread_create(GThreadFunc fn)
 {
     GThread *thread;
 #ifndef _WIN32
     sigset_t set, oldset;

     sigfillset(&set);
     pthread_sigmask(SIG_SETMASK, &set, &oldset);
 #endif

     thread = g_thread_new("trace-thread", fn, NULL);

 #ifndef _WIN32
     pthread_sigmask(SIG_SETMASK, &oldset, NULL);
 #endif

     return thread;
 }

 bool st_init(void)
 {
     GThread *thread;

     trace_pid = getpid();

     thread = trace_thread_create(writeout_thread);
     if (!thread) {
         warn_report("unable to initialize simple trace backend");
         return false;
     }

     atexit(st_flush_trace_buffer);
     return true;
 }

 void st_init_group(size_t group)
 {
     TraceEventIter iter;

     if (!trace_writeout_enabled) {
         return;
     }

     trace_event_iter_init_group(&iter, group);
     st_write_event_mapping(&iter);
 }
	/*
	* Simple trace backend
	*
	* Copyright IBM, Corp. 2010
	*
	* This work is licensed under the terms of the GNU GPL, version 2. See
	* the COPYING file in the top-level directory.
	*
	*/

	#include "qemu/osdep.h"
	#ifndef _WIN32
	#include <pthread.h>
	#endif
	#include "qemu/timer.h"
	#include "trace/control.h"
	#include "trace/simple.h"
	#include "qemu/error-report.h"
	#include "qemu/qemu-print.h"

	/** Trace file header event ID, picked to avoid conflict with real event IDs */
	#define HEADER_EVENT_ID (~(uint64_t)0)

	/** Trace file magic number */
	#define HEADER_MAGIC 0xf2b177cb0aa429b4ULL

	/** Trace file version number, bump if format changes */
	#define HEADER_VERSION 4

	/** Records were dropped event ID */
	#define DROPPED_EVENT_ID (~(uint64_t)0 - 1)

	/** Trace record is valid */
	#define TRACE_RECORD_VALID ((uint64_t)1 << 63)

	/*
	* Trace records are written out by a dedicated thread. The thread waits for
	* records to become available, writes them out, and then waits again.
	*/
	static GMutex trace_lock;
	static GCond trace_available_cond;
	static GCond trace_empty_cond;

	static bool trace_available;
	static bool trace_writeout_enabled;

	enum {
	TRACE_BUF_LEN = 4096 * 64,
	TRACE_BUF_FLUSH_THRESHOLD = TRACE_BUF_LEN / 4,
	};

	uint8_t trace_buf[TRACE_BUF_LEN];
	static volatile gint trace_idx;
	static unsigned int writeout_idx;
	static volatile gint dropped_events;
	static uint32_t trace_pid;
	static FILE *trace_fp;
	static char *trace_file_name;

	#define TRACE_RECORD_TYPE_MAPPING 0
	#define TRACE_RECORD_TYPE_EVENT 1

	/* * Trace buffer entry */
	typedef struct {
	uint64_t event; /* event ID value */
	uint64_t timestamp_ns;
	uint32_t length; /* in bytes */
	uint32_t pid;
	uint64_t arguments[];
	} TraceRecord;

	typedef struct {
	uint64_t header_event_id; /* HEADER_EVENT_ID */
	uint64_t header_magic; /* HEADER_MAGIC */
	uint64_t header_version; /* HEADER_VERSION */
	} TraceLogHeader;


	static void read_from_buffer(unsigned int idx, void *dataptr, size_t size);
	static unsigned int write_to_buffer(unsigned int idx, void *dataptr, size_t size);

	static void clear_buffer_range(unsigned int idx, size_t len)
	{
	uint32_t num = 0;
	while (num < len) {
	if (idx >= TRACE_BUF_LEN) {
	idx = idx % TRACE_BUF_LEN;
	}
	trace_buf[idx++] = 0;
	num++;
	}
	}
	/**
	* Read a trace record from the trace buffer
	*
	* @idx Trace buffer index
	* @record Trace record to fill
	*
	* Returns false if the record is not valid.
	*/
	static bool get_trace_record(unsigned int idx, TraceRecord **recordptr)
	{
	uint64_t event_flag = 0;
	TraceRecord record;
	/* read the event flag to see if its a valid record */
	read_from_buffer(idx, &record, sizeof(event_flag));

	if (!(record.event & TRACE_RECORD_VALID)) {
	return false;
	}

	smp_rmb(); /* read memory barrier before accessing record */
	/* read the record header to know record length */
	read_from_buffer(idx, &record, sizeof(TraceRecord));
	recordptr = malloc(record.length); / don't use g_malloc, can deadlock when traced */
	/* make a copy of record to avoid being overwritten */
	read_from_buffer(idx, *recordptr, record.length);
	smp_rmb(); /* memory barrier before clearing valid flag */
	(*recordptr)->event &= ~TRACE_RECORD_VALID;
	/* clear the trace buffer range for consumed record otherwise any byte
	* with its MSB set may be considered as a valid event id when the writer
	* thread crosses this range of buffer again.
	*/
	clear_buffer_range(idx, record.length);
	return true;
	}

	/**
	* Kick writeout thread
	*
	* @wait Whether to wait for writeout thread to complete
	*/
	static void flush_trace_file(bool wait)
	{
	g_mutex_lock(&trace_lock);
	trace_available = true;
	g_cond_signal(&trace_available_cond);

	if (wait) {
	g_cond_wait(&trace_empty_cond, &trace_lock);
	}

	g_mutex_unlock(&trace_lock);
	}

	static void wait_for_trace_records_available(void)
	{
	g_mutex_lock(&trace_lock);
	while (!(trace_available && trace_writeout_enabled)) {
	g_cond_signal(&trace_empty_cond);
	g_cond_wait(&trace_available_cond, &trace_lock);
	}
	trace_available = false;
	g_mutex_unlock(&trace_lock);
	}

	static gpointer writeout_thread(gpointer opaque)
	{
	TraceRecord *recordptr;
	union {
	TraceRecord rec;
	uint8_t bytes[sizeof(TraceRecord) + sizeof(uint64_t)];
	} dropped;
	unsigned int idx = 0;
	int dropped_count;
	size_t unused __attribute__ ((unused));
	uint64_t type = TRACE_RECORD_TYPE_EVENT;

	for (;;) {
	wait_for_trace_records_available();

	if (g_atomic_int_get(&dropped_events)) {
	dropped.rec.event = DROPPED_EVENT_ID;
	dropped.rec.timestamp_ns = get_clock();
	dropped.rec.length = sizeof(TraceRecord) + sizeof(uint64_t);
	dropped.rec.pid = trace_pid;
	do {
	dropped_count = g_atomic_int_get(&dropped_events);
	} while (!g_atomic_int_compare_and_exchange(&dropped_events,
	dropped_count, 0));
	dropped.rec.arguments[0] = dropped_count;
	unused = fwrite(&type, sizeof(type), 1, trace_fp);
	unused = fwrite(&dropped.rec, dropped.rec.length, 1, trace_fp);
	}

	while (get_trace_record(idx, &recordptr)) {
	unused = fwrite(&type, sizeof(type), 1, trace_fp);
	unused = fwrite(recordptr, recordptr->length, 1, trace_fp);
	writeout_idx += recordptr->length;
	free(recordptr); /* don't use g_free, can deadlock when traced */
	idx = writeout_idx % TRACE_BUF_LEN;
	}

	fflush(trace_fp);
	}
	return NULL;
	}

	void trace_record_write_u64(TraceBufferRecord *rec, uint64_t val)
	{
	rec->rec_off = write_to_buffer(rec->rec_off, &val, sizeof(uint64_t));
	}

	void trace_record_write_str(TraceBufferRecord rec, const char s, uint32_t slen)
	{
	/* Write string length first */
	rec->rec_off = write_to_buffer(rec->rec_off, &slen, sizeof(slen));
	/* Write actual string now */
	rec->rec_off = write_to_buffer(rec->rec_off, (void*)s, slen);
	}

	int trace_record_start(TraceBufferRecord *rec, uint32_t event, size_t datasize)
	{
	unsigned int idx, rec_off, old_idx, new_idx;
	uint32_t rec_len = sizeof(TraceRecord) + datasize;
	uint64_t event_u64 = event;
	uint64_t timestamp_ns = get_clock();

	do {
	old_idx = g_atomic_int_get(&trace_idx);
	smp_rmb();
	new_idx = old_idx + rec_len;

	if (new_idx - writeout_idx > TRACE_BUF_LEN) {
	/* Trace Buffer Full, Event dropped ! */
	g_atomic_int_inc(&dropped_events);
	return -ENOSPC;
	}
	} while (!g_atomic_int_compare_and_exchange(&trace_idx, old_idx, new_idx));

	idx = old_idx % TRACE_BUF_LEN;

	rec_off = idx;
	rec_off = write_to_buffer(rec_off, &event_u64, sizeof(event_u64));
	rec_off = write_to_buffer(rec_off, &timestamp_ns, sizeof(timestamp_ns));
	rec_off = write_to_buffer(rec_off, &rec_len, sizeof(rec_len));
	rec_off = write_to_buffer(rec_off, &trace_pid, sizeof(trace_pid));

	rec->tbuf_idx = idx;
	rec->rec_off = (idx + sizeof(TraceRecord)) % TRACE_BUF_LEN;
	return 0;
	}

	static void read_from_buffer(unsigned int idx, void *dataptr, size_t size)
	{
	uint8_t *data_ptr = dataptr;
	uint32_t x = 0;
	while (x < size) {
	if (idx >= TRACE_BUF_LEN) {
	idx = idx % TRACE_BUF_LEN;
	}
	data_ptr[x++] = trace_buf[idx++];
	}
	}

	static unsigned int write_to_buffer(unsigned int idx, void *dataptr, size_t size)
	{
	uint8_t *data_ptr = dataptr;
	uint32_t x = 0;
	while (x < size) {
	if (idx >= TRACE_BUF_LEN) {
	idx = idx % TRACE_BUF_LEN;
	}
	trace_buf[idx++] = data_ptr[x++];
	}
	return idx; /* most callers wants to know where to write next */
	}

	void trace_record_finish(TraceBufferRecord *rec)
	{
	TraceRecord record;
	read_from_buffer(rec->tbuf_idx, &record, sizeof(TraceRecord));
	smp_wmb(); /* write barrier before marking as valid */
	record.event \|= TRACE_RECORD_VALID;
	write_to_buffer(rec->tbuf_idx, &record, sizeof(TraceRecord));

	if (((unsigned int)g_atomic_int_get(&trace_idx) - writeout_idx)
	> TRACE_BUF_FLUSH_THRESHOLD) {
	flush_trace_file(false);
	}
	}

	static int st_write_event_mapping(TraceEventIter *iter)
	{
	uint64_t type = TRACE_RECORD_TYPE_MAPPING;
	TraceEvent *ev;

	while ((ev = trace_event_iter_next(iter)) != NULL) {
	uint64_t id = trace_event_get_id(ev);
	const char *name = trace_event_get_name(ev);
	uint32_t len = strlen(name);
	if (fwrite(&type, sizeof(type), 1, trace_fp) != 1 \|\|
	fwrite(&id, sizeof(id), 1, trace_fp) != 1 \|\|
	fwrite(&len, sizeof(len), 1, trace_fp) != 1 \|\|
	fwrite(name, len, 1, trace_fp) != 1) {
	return -1;
	}
	}

	return 0;
	}

	/**
	* Enable / disable tracing, return whether it was enabled.
	*
	* @enable: enable if %true, else disable.
	*/
	bool st_set_trace_file_enabled(bool enable)
	{
	TraceEventIter iter;
	bool was_enabled = trace_fp;

	if (enable == !!trace_fp) {
	return was_enabled; /* no change */
	}

	/* Halt trace writeout */
	flush_trace_file(true);
	trace_writeout_enabled = false;
	flush_trace_file(true);

	if (enable) {
	static const TraceLogHeader header = {
	.header_event_id = HEADER_EVENT_ID,
	.header_magic = HEADER_MAGIC,
	/* Older log readers will check for version at next location */
	.header_version = HEADER_VERSION,
	};

	trace_fp = fopen(trace_file_name, "wb");
	if (!trace_fp) {
	return was_enabled;
	}

	trace_event_iter_init_all(&iter);
	if (fwrite(&header, sizeof header, 1, trace_fp) != 1 \|\|
	st_write_event_mapping(&iter) < 0) {
	fclose(trace_fp);
	trace_fp = NULL;
	return was_enabled;
	}

	/* Resume trace writeout */
	trace_writeout_enabled = true;
	flush_trace_file(false);
	} else {
	fclose(trace_fp);
	trace_fp = NULL;
	}
	return was_enabled;
	}

	/**
	* Set the name of a trace file
	*
	* @file The trace file name or NULL for the default name-<pid> set at
	* config time
	*/
	void st_set_trace_file(const char *file)
	{
	bool saved_enable = st_set_trace_file_enabled(false);

	g_free(trace_file_name);

	if (!file) {
	/* Type cast needed for Windows where getpid() returns an int. */
	trace_file_name = g_strdup_printf(CONFIG_TRACE_FILE "-" FMT_pid, (pid_t)getpid());
	} else {
	trace_file_name = g_strdup_printf("%s", file);
	}

	st_set_trace_file_enabled(saved_enable);
	}

	void st_print_trace_file_status(void)
	{
	qemu_printf("Trace file \"%s\" %s.\n",
	trace_file_name, trace_fp ? "on" : "off");
	}

	void st_flush_trace_buffer(void)
	{
	flush_trace_file(true);
	}

	/* 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
	* does not steal signals when the rest of the program wants them blocked.
	*/
	static GThread *trace_thread_create(GThreadFunc fn)
	{
	GThread *thread;
	#ifndef _WIN32
	sigset_t set, oldset;

	sigfillset(&set);
	pthread_sigmask(SIG_SETMASK, &set, &oldset);
	#endif

	thread = g_thread_new("trace-thread", fn, NULL);

	#ifndef _WIN32
	pthread_sigmask(SIG_SETMASK, &oldset, NULL);
	#endif

	return thread;
	}

	bool st_init(void)
	{
	GThread *thread;

	trace_pid = getpid();

	thread = trace_thread_create(writeout_thread);
	if (!thread) {
	warn_report("unable to initialize simple trace backend");
	return false;
	}

	atexit(st_flush_trace_buffer);
	return true;
	}

	void st_init_group(size_t group)
	{
	TraceEventIter iter;

	if (!trace_writeout_enabled) {
	return;
	}

	trace_event_iter_init_group(&iter, group);
	st_write_event_mapping(&iter);
	}