Re: [PATCH v4 4/7] contrib/plugins/uftrace: add timestamp-based-on-real-time option

[Pierrick Bouvier]

On 8/5/25 3:23 AM, Alex Bennée wrote:
Pierrick Bouvier <pierrick.bouv...@linaro.org> writes:
usage: timestamp-based-on-real-time=[on|off]

Instead of using number of instructions executed (which is per vcpu), we
use the wall time for timestamps. This is useful when tracing user mode
programs as well.

Arguably it is the more useful default when examining flame graphs. The
instruction count is entirely artificial and wouldn't map onto real
execution time anyway.

Maybe use-icount-for-ts and default to off?


I don't mind changing the default if you prefer so.


Signed-off-by: Pierrick Bouvier <pierrick.bouv...@linaro.org>
---
  contrib/plugins/uftrace.c | 29 +++++++++++++++++++++++++++++
  1 file changed, 29 insertions(+)

diff --git a/contrib/plugins/uftrace.c b/contrib/plugins/uftrace.c
index bd4219db693..9211f273700 100644
--- a/contrib/plugins/uftrace.c
+++ b/contrib/plugins/uftrace.c
@@ -94,8 +94,28 @@ enum uftrace_record_type {
  static struct qemu_plugin_scoreboard *score;
  static uint64_t trace_sample;
  static bool trace_privilege_level;
+static bool timestamp_based_on_real_time;
  static CpuOps arch_ops;
  
+static uint64_t gettime_ns(void)
+{
+#ifdef _WIN32
+    /*
+     * On Windows, timespec_get is available only with UCRT, but not with
+     * MinGW64 environment. Simplify by using only gettimeofday on this
+     * platform.
+     */
+    struct timeval tv;
+    gettimeofday(&tv, NULL);
+    uint64_t now_ns = tv.tv_sec * 1000 * 1000 * 1000 + tv.tv_usec * 1000;
+#else
+    struct timespec ts;
+    timespec_get(&ts, TIME_UTC);
+    uint64_t now_ns = ts.tv_sec * 1000 * 1000 * 1000 + ts.tv_nsec;

Why use timspec_get here, QEMU's own timer uses:

   /* get host real time in nanosecond */
   static inline int64_t get_clock_realtime(void)
   {
       struct timeval tv;

       gettimeofday(&tv, NULL);
       return tv.tv_sec * 1000000000LL + (tv.tv_usec * 1000);
   }


We need nanoseconds timestamps (which timespec offers and not 
gettimeofday), or we lose precision for short lived functions, resulting 
in bad shaped traces.


+#endif
+    return now_ns;
+}
+
  static void uftrace_write_map(bool system_emulation)
  {
      const char *path = "./uftrace.data/sid-0.map";
@@ -454,6 +474,9 @@ static void cpu_set_new_sample(Cpu *cpu, uint64_t timestamp)
  
  static uint64_t cpu_get_timestamp(const Cpu *cpu)
  {
+    if (timestamp_based_on_real_time) {
+        return gettime_ns();
+    }
      return cpu->insn_count;
  }
  
@@ -866,6 +889,12 @@ QEMU_PLUGIN_EXPORT int qemu_plugin_install(qemu_plugin_id_t id,
                  fprintf(stderr, "boolean argument parsing failed: %s\n", opt);
                  return -1;
              }
+        } else if (g_strcmp0(tokens[0], "timestamp-based-on-real-time") == 0) {
+            if (!qemu_plugin_bool_parse(tokens[0], tokens[1],
+                                        &timestamp_based_on_real_time)) {
+                fprintf(stderr, "boolean argument parsing failed: %s\n", opt);
+                return -1;
+            }
          } else {
              fprintf(stderr, "option parsing failed: %s\n", opt);
              return -1;