qemu: expose rr

run: expose forgotten -Q, document it

Author: cirosantilli

README.adoc

@@ -7011,19 +7011,70 @@ TODO do even more awesome offline post-mortem analysis things, such as:
 
 ==== QEMU record and replay
 
-QEMU supports deterministic record and replay by saving external inputs, which would be awesome to understand the kernel, as you would be able to examine a single run as many times as you would like.
+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:
 
-This mechanism first requires a trace to be generated on an initial record run. The trace is then used on the replay runs to make them deterministic.
+This awesome feature allows you to examine a single run as many times as you would like until you understand everything:
 
-Unfortunately it is not working in the current QEMU: https://stackoverflow.com/questions/46970215/how-to-use-qemus-deterministic-record-and-replay-feature-for-a-linux-kernel-boo
+....
+# Record a run.
+./run -F '/rand_check.out;/poweroff.out;' -r
+# Replay the run.
+./run -F '/rand_check.out;/poweroff.out;' -R
+....
+
+By comparing the terminal output of both runs, we can see that they are the exact same, including things which normally differ across runs:
+
+* timestamps of dmesg output
+* <<rand_check-out>> output
 
-Patches were merged in post v2.12.0-rc2 but it crashed for me and I opened a minimized bug report: https://bugs.launchpad.net/qemu/+bug/1762179
+The record and replay feature was revived around QEMU v3.0.0. It existed earlier but it rot completely. As of v3.0.0 it is still flaky: sometimes we get deadlocks, and only a limited number of command line arguments are supported.
 
-We don't expose record and replay on our scripts yet since it was was not very stable, but we will do so when it stabilizes.
+Documented at: https://github.com/qemu/qemu/blob/v2.12.0/docs/replay.txt
+
+TODO: using `-r` as above leads to a kernel warning:
+
+....
+rcu_sched detected stalls on CPUs/tasks
+....
 
-<<rand_check-out>> is a good way to test out if record and replay is actually deterministic.
+TODO: replay deadlocks intermittently at disk operations, last kernel message:
+
+....
+EXT4-fs (sda): re-mounted. Opts: block_validity,barrier,user_xattr
+....
+
+TODO replay with network gets stuck:
+
+....
+./run -F '/sbin/ifup -a;wget -S google.com;/poweroff.out;' -r
+./run -F '/sbin/ifup -a;wget -S google.com;/poweroff.out;' -R
+....
+
+after the message:
+
+....
+adding dns 10.0.2.3
+....
+
+There is explicit network support on the QEMU patches, but either it is buggy or we are not using the correct magic options.
+
+TODO `arm` and `aarch64` only seem to work with initrd since I cannot plug a working IDE disk device? See also: https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg05245.html
+
+Then, when I tried with <<initrd>> and no disk:
+
+....
+./build -aA -i
+./run -aA -F '/rand_check.out;/poweroff.out;' -i -r
+./run -aA -F '/rand_check.out;/poweroff.out;' -i -R
+....
+
+QEMU crashes with:
+
+....
+ERROR:replay/replay-time.c:49:replay_read_clock: assertion failed: (replay_file && replay_mutex_locked())
+....
 
-Alternatively, https://github.com/mozilla/rr[`mozilla/rr`] claims it is able to run QEMU: but using it would require you to step through QEMU code itself. Likely doable, but do you really want to?
+I had the same error previously on x86-64, but it was fixed: https://bugs.launchpad.net/qemu/+bug/1762179 so maybe the forgot to fix it for `aarch64`?
 
 ==== QEMU trace multicore
 

build-usage.adoc

@@ -35,6 +35,7 @@
                              on top of it.
 |`-M` |`VARIANT`            |gem5 build variant.
 |`-p` |                     |Pass extra arguments to the `rootfs_post_build_script`.
+|`-Q` |`VARIANT``           |QEMU build variant.
 |`-S` |                     |Don't build QEMU with SDL support.
                              Graphics such as X11 won't work, only the terminal.
 |`-s` |                     |Add a custom suffix to the build.

common

@@ -69,7 +69,8 @@ set_common_vars() {
   common_images_dir="${buildroot_out_dir}/images"
   host_dir="${buildroot_out_dir}/host"
   common_qemu_run_dir="${out_arch_dir}/qemu/${common_run_id}"
-  common_qemu_termout_file="${common_qemu_run_dir}/termout.txt"
+    common_qemu_termout_file="${common_qemu_run_dir}/termout.txt"
+    common_qemu_rrfile="${common_qemu_run_dir}/rrfile"
   common_linux_custom_dir="${build_dir}/linux-custom"
   common_linux_variant_dir="${common_linux_custom_dir}.${linux_variant}"
   common_qemu_custom_dir="${build_dir}/host-qemu-custom"

run

@@ -27,6 +27,7 @@ initramfs=false
 memory=256M
 nographic=true
 prebuilt=false
+rr=
 root=
 tmux=false
 tmux_args=
@@ -35,7 +36,7 @@ trace_enabled=false
 # just to prevent QEMU from emitting a warning that '' is not valid.
 trace_type=pr_manager_run
 vnc=
-while getopts a:c:DdE:e:F:f:G:ghIiKkL:M:m:N:n:PT:t:U:uVX:x OPT; do
+while getopts a:c:DdE:e:F:f:G:ghIiKkL:M:m:N:n:PQ:RrT:t:U:uVX:x OPT; do
   case "$OPT" in
     a)
       arch="$OPTARG"
@@ -108,6 +109,15 @@ while getopts a:c:DdE:e:F:f:G:ghIiKkL:M:m:N:n:PT:t:U:uVX:x OPT; do
     P)
       prebuilt=true
       ;;
+    Q)
+      common_qemu_variant="$OPTARG"
+      ;;
+    R)
+      rr=replay
+      ;;
+    r)
+      rr=record
+      ;;
     T)
       trace_enabled=true
       trace_type="$OPTARG"
@@ -209,7 +219,7 @@ ${gem5opts} \
 --dtb "${common_gem5_system_dir}/arm/dt/armv8_gem5_v1_big_little_2_2.dtb" \\
 --kernel="${common_vmlinux}" \\
 --little-cpus=2 \\
-${extra_flags} \\
+${extra_flags} \
 "
   else
     gem5_common="\
@@ -282,16 +292,39 @@ ${vnc}"
     extra_flags="${extra_flags} -initrd '${common_images_dir}/rootfs.cpio' \\
 "
   fi
+
+  # Disk related options.
   if "$ramfs"; then
     # TODO why is this needed, and why any string works.
     root='root=/dev/anything'
   else
     if [ ! "$arch" = mips64 ]; then
-      extra_flags="${extra_flags} -drive 'file=${common_images_dir}/rootfs.ext2.qcow2,format=qcow2,if=virtio,snapshot' \\
+      if [ -n "$rr" ]; then
+        driveif=none
+        rrid=',id=img-direct'
+        root='root=/dev/sda'
+      else
+        driveif=virtio
+        root='root=/dev/vda'
+        rrid=
+      fi
+      extra_flags="${extra_flags} -drive 'file=${common_images_dir}/rootfs.ext2.qcow2,format=qcow2,if=${driveif},snapshot${rrid}' \\
+"
+      if [ -n "$rr" ]; then
+        extra_flags="${extra_flags} \\
+-drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay \\
+-device ide-hd,drive=img-blkreplay \\
 "
-      root='root=/dev/vda'
+      fi
     fi
   fi
+
+  if [ -n "$rr" ]; then
+    extra_flags="${extra_flags} \
+-object filter-replay,id=replay,netdev=net0 \\
+-icount 'shift=7,rr=${rr},rrfile=${common_qemu_rrfile}' \\
+"
+  fi
   case "$arch" in
     x86_64)
       if "$kgdb"; then
@@ -342,7 +375,8 @@ ${extra_flags} \
     mips64)
       if ! "$ramfs"; then
         root='root=/dev/hda'
-        extra_flags="${extra_flags} -drive 'file=${common_images_dir}/rootfs.ext2.qcow2,format=qcow2,snapshot' \\
+        extra_flags="${extra_flags} \
+-drive 'file=${common_images_dir}/rootfs.ext2.qcow2,format=qcow2,snapshot' \\
 "
       fi
       cmd="\
@@ -359,7 +393,7 @@ fi
 if "$tmux"; then
   if "$gem5"; then
     eval "./tmu 'sleep 2;./gem5-shell -n ${common_run_id} ${tmux_args};'"
-  elif  "$debug"; then
+  elif "$debug"; then
     eval "./tmu ./rungdb -a '${arch} -L ${common_linux_variant}' -n ${common_run_id} ${tmux_args}"
   fi
 fi

run-usage.adoc

@@ -44,7 +44,10 @@
                         Any
 |`-N`   |`VARIANT`     |gem5 source input variant.
 |`-n`   |              |Run ID.
+|`-R`   |              |Replay a QEMU run record deterministically.
+|`-r`   |              |Record a QEMU run record for later replay with `-R`.
 |`-P`   |              |Run the downloaded prebuilt images.
+|`-Q`   |`VARIANT``    |QEMU build variant.
 |`-T`   |`TRACE_TYPES` |Set trace events to be enabled.
                         If not given, gem5 tracing is completely disabled, while QEMU tracing
                         is enabled but uses default traces that are very rare and don't affect