run: trace to stdout

README.adoc

@@ -8351,7 +8351,7 @@ TODO do even more awesome offline post-mortem analysis things, such as:
 
 ==== QEMU record and replay
 
-QEMU runs are not deterministic by default, however it does support a record and replay mechanism that allows you to replay a previous run deterministically:
+QEMU runs, unlike gem5, are not deterministic by default, however it does support a record and replay mechanism that allows you to replay a previous run deterministically.
 
 This awesome feature allows you to examine a single run as many times as you would like until you understand everything:
 
@@ -8410,8 +8410,8 @@ TODO `arm` and `aarch64` only seem to work with initrd since I cannot plug a wor
 Then, when I tried with <<initrd>> and no disk:
 
 ....
-./build-buildroot --arch aarch64 -i
+./build-buildroot --arch aarch64 --initrd
-./qemu-rr --arch aarch64 --eval-after '/rand_check.out;/poweroff.out;' -i
+./qemu-rr --arch aarch64 --eval-after '/rand_check.out;/poweroff.out;' --initrd
 ....
 
 QEMU crashes with:
@@ -8466,15 +8466,21 @@ just appears to output both cores intertwined without any clear differentiation.
 
 ==== gem5 tracing
 
-gem5 unlike QEMU is deterministic by default without needing to replay traces
+gem5 provides also provides a tracing mechanism documented at: link:http://www.gem5.org/Trace_Based_Debugging[]:
-
-But it also provides a tracing mechanism documented at: link:http://www.gem5.org/Trace_Based_Debugging[] to allow easily inspecting certain aspects of the system:
 
 ....
 ./run --arch aarch64 --eval 'm5 exit' --gem5 --trace Exec
 less "$(./getvar --arch aarch64 run_dir)/trace.txt"
 ....
 
+Output the trace to stdout instead of a file:
+
+....
+./run --arch aarch64 --eval 'm5 exit' --gem5 --trace Exec --trace-stdout
+....
+
+This would produce a lot of output however, so you will likely not want that when tracing a Linux kernel boot instructions. But it can be very convenient for smaller traces.
+
 List all available debug flags:
 
 ....
@@ -8488,6 +8494,8 @@ less "$(./getvar gem5_src_dir)/src/cpu/SConscript"
 less "$(./getvar gem5_src_dir)/src/cpu/exetrace.cc"
 ....
 
+The traces are generated from `DPRINTF(<trace-id>` calls scattered throughout the code.
+
 As can be seen on the `Sconstruct`, `Exec` is just an alias that enables a set of flags.
 
 Be warned, the trace is humongous, at 16Gb.
@@ -11779,6 +11787,11 @@ We have some link:https://github.com/pexpect/pexpect[pexpect] automated tests fo
 ./test-gdb
 ....
 
+Sources:
+
+* link:build-test-gdb[]
+* link:test-gdb[]
+
 Not all of them  are passing right now due to: <<gem5-gdb-step-debug-kernel-aarch64>>.
 
 If something goes wrong, re-run the test commands manually and use `--verbose` to understand what happened:
@@ -11794,10 +11807,19 @@ and possibly repeat the GDB steps manually with the usual:
 ./run-gdb --arch arm --baremetal add --no-continue --verbose
 ....
 
-Sources:
+To debug GDB problems on gem5, you might want to enable the following <<gem5-tracing,tracing>> options:
 
-* link:build-test-gdb[]
+....
-* link:test-gdb[]
+./run \
+  --arch arm \
+  --baremetal add \
+  --wait-gdb \
+  --trace GDBRecv,GDBSend \
+  --trace-stdout \
+;
+....
+
+====== Test GDB Linux kernel
 
 For the Linux kernel, do the following manual tests for now.
 

run

@@ -35,6 +35,7 @@ defaults = {
     'terminal': False,
     'tmux': None,
     'trace': None,
+    'trace_stdout': False,
     'userland': None,
     'userland_before': '',
     'vnc': False,
@@ -151,10 +152,14 @@ def main(args, extra_args=None):
         extra_env['M5_PATH'] = common.gem5_system_dir
         # https://stackoverflow.com/questions/52312070/how-to-modify-a-file-under-src-python-and-run-it-without-rebuilding-in-gem5/52312071#52312071
         extra_env['M5_OVERRIDE_PY_SOURCE'] = 'true'
+        if args.trace_stdout:
+            debug_file = 'cout'
+        else:
+            debug_file = 'trace.txt'
         cmd.extend(
             [
                 common.executable, common.Newline,
-                '--debug-file=trace.txt', common.Newline,
+                '--debug-file', debug_file, common.Newline,
                 '--listener-mode', 'on', common.Newline,
                 '--outdir', common.m5out_dir, common.Newline,
             ] +
@@ -553,6 +558,12 @@ disabled, while QEMU tracing is enabled but uses default traces that are very
 rare and don't affect performance, because `./configure
 --enable-trace-backends=simple` seems to enable some traces by default, e.g.
 `pr_manager_run`, and I don't know how to get rid of them.
+'''
+    )
+    parser.add_argument(
+        '--trace-stdout', default=defaults['trace_stdout'], action='store_true',
+        help='''\
+Output trace to stdout instead of a file. Only works for gem5 currently.
 '''
     )
     init_group.add_argument(