trace2line: gem5 support

As noted however, it is potentially too slow to be useful.

run: unify gem5 and qemu tracing under -T

readme: overhaul tracing documentation from what I've learnt from trace2line

README.adoc

@@ -1150,6 +1150,8 @@ Maybe it is because they are being copied around at specific locations instead o
 
 See also: https://stackoverflow.com/questions/2589845/what-are-the-first-operations-that-the-linux-kernel-executes-on-boot
 
+<<gem5-tracing>> with `--debug-flags=Exec` does show the right symbols however! So in the worst case, we can just read their source. Amazing.
+
 ==== GDB step debug early boot by address
 
 One possibility is to run:
@@ -3353,9 +3355,9 @@ You can still send key presses to QEMU however even without the mouse capture, j
 
 === Tracing
 
-QEMU has a mechanism to log all instructions executed to a file.
+QEMU can log several different events.
 
-To do it for the Linux kernel boot we have a helper:
+The most interesting are events which show instructions that QEMU ran, for which we have a helper:
 
 ....
 ./trace-boot -a x86_64
@@ -3367,6 +3369,20 @@ You can then inspect the instructions with:
 less ./out/x86_64/qemu/trace.txt
 ....
 
+Enable other specific trace events:
+
+....
+./run -T trace1,trace2
+./qemu-trace2txt -a "$arch"
+less ./out/x86_64/qemu/trace.txt
+....
+
+Get the list of available trace events:
+
+....
+./run -T help
+....
+
 This functionality relies on the following setup:
 
 * `./configure --enable-trace-backends=simple`. This logs in a binary format to the trace file.
@@ -3395,8 +3411,7 @@ in which the boot appears to hang for a considerable time.
 We can further use Binutils' `addr2line` to get the line that corresponds to each address:
 
 ....
-./trace-boot -a x86_64
+./trace-boot -a x86_64 && ./trace2line -a x86_64
-./trace2line -a x86_64
 less ./out/x86_64/qemu/trace-lines.txt
 ....
 
@@ -3435,43 +3450,66 @@ Alternatively, https://github.com/mozilla/rr[`mozilla/rr`] claims it is able to
 
 gem5 also has a tracing mechanism, as documented at: http://www.gem5.org/Trace_Based_Debugging
 
-Try it out with:
-
-....
-./run -a aarch64 -E 'm5 exit' -G '--debug-flags=Exec' -g
-....
-
-The trace file is located at:
-
 ....
+./run -a aarch64 -E 'm5 exit' -g -T Exec
 less out/aarch64/gem5/m5out/trace.txt
 ....
 
-but be warned, it is humongous, at 16Gb.
+List all available debug flags:
-
-It made the runtime about 4x slower on the <<p51>>, with or without `.gz` compression.
-
-The list of available debug flags can be found with:
 
 ....
 ./run -a aarch64 -G --debug-help -g
 ....
 
-but for meaningful descriptions you need to look at the source code:
+but to understand most of them you have to look at the source code:
 
 ....
 less gem5/gem5/src/cpu/SConscript
+less gem5/gem5/src/cpu/exetrace.cc
 ....
 
-The default `Exec` format reads symbol names from the Linux kernel image and show them, which is pretty awesome if you ask me.
+As can be seen on the `Sconstruct`, `Exec` is just an alias that enables a set of flags.
 
-TODO can we get just the executed addresses out of gem5? The following gets us closer, but not quite:
+Be warned, the trace is humongous, at 16Gb.
+
+We can make the trace smaller by naming the trace file as `trace.txt.gz`, which enables GZIP compression, but that is not currently exposed on our scripts, since you usually just need something human readable to work on.
+
+Enabling tracing made the runtime about 4x slower on the <<p51>>, with or without `.gz` compression.
+
+The output format is of type:
 
 ....
-./run -a aarch64 -E 'm5 exit' -G '--debug-flags=ExecEnable,ExecKernel,ExecUse' -g
+25007000: system.cpu T0 : @start_kernel    : stp
+25007000: system.cpu T0 : @start_kernel.0  :   addxi_uop   ureg0, sp, #-112 : IntAlu :  D=0xffffff8008913f90
+25007500: system.cpu T0 : @start_kernel.1  :   strxi_uop   x29, [ureg0] : MemWrite :  D=0x0000000000000000 A=0xffffff8008913f90
+25008000: system.cpu T0 : @start_kernel.2  :   strxi_uop   x30, [ureg0, #8] : MemWrite :  D=0x0000000000000000 A=0xffffff8008913f98
+25008500: system.cpu T0 : @start_kernel.3  :   addxi_uop   sp, ureg0, #0 : IntAlu :  D=0xffffff8008913f90
 ....
 
-We could of course just pipe it to stdout and `awk` it up.
+There are two types of lines:
+
+* full instructions, as the first line. Only shown if the `ExecMacro` flag is given.
+* micro ops that constitute the instruction, the lines that follow. Yes, `aarch64` also has microops: link:https://superuser.com/questions/934752/do-arm-processors-like-cortex-a9-use-microcode/934755#934755[]. Only shown if the `ExecMicro` flag is given.
+
+Breakdown:
+
+* `25007500`: time count in some unit. Note how the microops execute at further timestamps.
+* `system.cpu`: distinguishes between CPUs when there are more than one
+* `T0`: thread number. TODO: link:https://superuser.com/questions/133082/hyper-threading-and-dual-core-whats-the-difference/995858#995858[hyperthread]? How to play with it?
+* `@start_kernel`: we are in the `start_kernel` function. Awesome feature! Implemented with libelf https://sourceforge.net/projects/elftoolchain/ copy pasted in-tree `ext/libelf`. To get raw addresses, remove the `ExecSymbol`, which is enabled by `Exec`.
+* `.1` as in `@start_kernel.1`: index of the microop
+* `stp`: instruction disassembly. Seems to use `.isa` files dispersed per arch, which is an in house format: http://gem5.org/ISA_description_system
+* `strxi_uop   x29, [ureg0]`: microop disassembly.
+* `MemWrite :  D=0x0000000000000000 A=0xffffff8008913f90`: TODO. Further description of the microops.
+
+Trace the source lines just like <<trace-source-lines,for QEMU>> with:
+
+....
+./trace-boot -a aarch64 -g && ./trace2line -a aarch64 -g
+less ./out/aarch64/gem5/trace-lines.txt
+....
+
+TODO: 7452d399290c9c1fc6366cdad129ef442f323564 `./trace2line` this is too slow and takes hours. QEMU's processing of 170k events takes 7 seconds. gem5's processing is analogous, but there are 140M events, so it should take 7000 seconds ~ 2 hours which seems consistent with what I observe, so maybe there is no way to speed this up... The workaround is to just use gem5's `ExecSymbol` to get function granularity, and then GDB individually if line detail is needed?
 
 === QEMU GUI is unresponsive
 
@@ -4287,6 +4325,24 @@ Pass options to the `gem5` executable itself:
 ./run -G '-h' -g
 ....
 
+=== gem5 exit after a number of instructions
+
+Quit the simulation after `1024` instructions:
+
+....
+./run -g -- -I 1024
+....
+
+Can be nicely checked with <<gem5-tracing>>.
+
+Cycles instead of instructions:
+
+....
+./run -g -- -m 1024
+....
+
+Otherwise the simulation runs forever by default.
+
 === Run multiple gem5 instances at once
 
 gem5 just assigns new ports if some ports are occupied, so we can do:

bench-boot

@@ -12,7 +12,7 @@ qemu_insts() (
   arch="$1"
   ./qemu-trace2txt -a "$arch"
   set_common_vars "$arch"
-  printf "instructions $(wc -l "${qemu_trace_txt_file}" | cut -d' ' -f1)\n" >> "$common_bench_boot"
+  printf "instructions $(wc -l "${common_trace_txt_file}" | cut -d' ' -f1)\n" >> "$common_bench_boot"
 )
 rm -f "${common_bench_boot}"
 

common

@@ -36,11 +36,12 @@ set_common_vars() {
   gem5_out_dir="${out_arch_dir}/gem5"
   m5out_dir="${gem5_out_dir}/m5out"
   qemu_out_dir="${out_arch_dir}/qemu"
-  qemu_trace_txt_file="${qemu_out_dir}/trace.txt"
   if "$gem5"; then
     common_out_run_dir="$gem5_out_dir"
+    common_trace_txt_file="${common_out_run_dir}/m5out/trace.txt"
   else
     common_out_run_dir="$qemu_out_dir"
+    common_trace_txt_file="${common_out_run_dir}/trace.txt"
   fi
 }
 root_dir="$(pwd)"

qemu-trace2txt

@@ -13,4 +13,4 @@ set_common_vars "$arch" false
 ./qemu/scripts/simpletrace.py \
   "${build_dir}/host-qemu-custom/trace-events-all" \
   "${qemu_out_dir}/trace.bin" \
-> "${qemu_trace_txt_file}"
+> "${common_trace_txt_file}"

run

@@ -28,9 +28,10 @@ nographic=true
 root=
 tmux=false
 tmux_args=
+trace_enabled=false
 # A dummy value that is already turned on by default and does not produce large output,
 # just to prevent QEMU from emitting a warning that '' is not valid.
-trace_enable=pr_manager_run
+trace_type=pr_manager_run
 vnc=
 while getopts a:c:DdE:e:F:f:G:ghIiKkm:T:U:uVx OPT; do
   case "$OPT" in
@@ -93,7 +94,8 @@ while getopts a:c:DdE:e:F:f:G:ghIiKkm:T:U:uVx OPT; do
       memory="$OPTARG"
       ;;
     T)
-      trace_enable="$OPTARG"
+      trace_enabled=true
+      trace_type="$OPTARG"
       ;;
     U)
       tmux_args="$OPTARG"
@@ -155,6 +157,10 @@ if "$gem5"; then
   else
     gem5_arch=ARM
   fi
+  if "$trace_enabled"; then
+    gem5opts="${gem5opts} --debug-flags='${trace_type}' \\
+"
+  fi
   gem5_common="\
 M5_PATH='${gem5_build_dir}/system' \\
 ${debug_vm} \
@@ -203,7 +209,7 @@ ${debug_vm} \
 -monitor telnet::45454,server,nowait \\
 -netdev user,hostfwd=tcp::45455-:45455,hostfwd=tcp::45456-:22,id=net0 \\
 -smp '${cpus}' \\
--trace 'enable=${trace_enable},file=${qemu_out_dir}/trace.bin' \\
+-trace 'enable=${trace_type},file=${qemu_out_dir}/trace.bin' \\
 -virtfs 'local,path=${p9_dir},mount_tag=host_scratch,security_model=mapped,id=host_scratch' \\
 -virtfs 'local,path=${buildroot_out_dir}/build,mount_tag=host_out,security_model=mapped,id=host_out' \\
 ${vnc}"

run-usage.adoc

@@ -40,11 +40,12 @@
                       The default is the minimum amount that boots all archs without extra
                       options added. Anything lower will lead some arch to fail to boot.
                       Any
-|`-T` |              |Enable extra QEMU trace events.
+|`-T` |`TRACE_TYPES` |Set trace events to be enabled.
-                      `./configure --enable-trace-backends=simple` seems to enable
+                      If not given, gem5 tracing is completely disabled, while QEMU tracing
-                      some by default, e.g. `pr_manager_run`, and I don't know how to
+                      is enabled but uses default traces that are very rare and don't affect
-                      get rid of them. But those events are rare and should not impact
+                      performance. `./configure --enable-trace-backends=simple` seems to enable
-                      performance, so we just ignore them for now.
+                      some traces by default, e.g. `pr_manager_run`, and I don't know how to
+                      get rid of them.
 |`-U` |              |Pass extra parameters to the program running on the `-u` tmux split.
 |`-u` |              |Create a tmUx split the window.
                       You must already be inside of a `tmux` session to use this option.

trace-boot

@@ -2,22 +2,30 @@
 set -eu
 . common
 set -- ${cli_trace_boot:-} "$@"
-while getopts a: OPT; do
+while getopts a:g OPT; do
   case "$OPT" in
     a)
       arch="$OPTARG"
       ;;
+    g)
+      gem5=true
+      ;;
   esac
 done
-set_common_vars "$arch" false
+shift "$(($OPTIND - 1))"
-time ./run -a "$arch" -e 'init=/poweroff.out' -T exec_tb
+set_common_vars "$arch" "$gem5"
-time ./qemu-trace2txt -a "$arch"
+if "$gem5"; then
-
+  # Exactly what Exec enabled, but without ExecSymbol and ExecMicro.
-# Instruction count.
+  time ./run -a aarch64 -E 'm5 exit' -g -T 'ExecEnable,ExecTicks,ExecOpClass,ExecThread,ExecEffAddr,ExecResult,ExecMacro,ExecFaulting,ExecUser,ExecKernel' "$@"
-# We could put this on a separate script, but it just adds more arch boilerplate to a new script.
+else
-# So let's just leave it here for now since it did not add a significant processing time.
+  time ./run -a "$arch" -e 'init=/poweroff.out' -T exec_tb "$@"
-echo "instructions $(wc -l "${qemu_trace_txt_file}" | cut -d' ' -f1)"
+  time ./qemu-trace2txt -a "$arch"
-entry_addr=$("${root_dir}/runtc" readelf -h "${build_dir}/linux-custom/vmlinux" | grep 'Entry point address' | sed -E 's/.*: *//')
+  # Instruction count.
-echo "entry_address ${entry_addr}"
+  # We could put this on a separate script, but it just adds more arch boilerplate to a new script.
-sed "/${entry_addr}/q" "${qemu_trace_txt_file}" >"${qemu_out_dir}/trace-boot.txt"
+  # So let's just leave it here for now since it did not add a significant processing time.
-echo "instructions_firmware $(wc -l "${qemu_out_dir}/trace-boot.txt" | cut -d' ' -f1)"
+  echo "instructions $(wc -l "${common_trace_txt_file}" | cut -d' ' -f1)"
+  entry_addr=$("${root_dir}/runtc" readelf -h "${build_dir}/linux-custom/vmlinux" | grep 'Entry point address' | sed -E 's/.*: *//')
+  echo "entry_address ${entry_addr}"
+  sed "/${entry_addr}/q" "${common_trace_txt_file}" >"${qemu_out_dir}/trace-boot.txt"
+  echo "instructions_firmware $(wc -l "${qemu_out_dir}/trace-boot.txt" | cut -d' ' -f1)"
+fi

trace2line

@@ -2,17 +2,26 @@
 set -eu
 . common
 set -- ${cli_trace2line:-} "$@"
-while getopts a: OPT; do
+while getopts a:g OPT; do
   case "$OPT" in
     a)
       arch="$OPTARG"
       ;;
+    g)
+      gem5=true
+      ;;
   esac
 done
-set_common_vars "$arch" false
+set_common_vars "$arch" "$gem5"
 kernel_dir="${build_dir}/linux-custom"
-sed -E 's/.*pc=//' "${qemu_out_dir}/trace.txt" | \
+(
+  if "$gem5"; then
+    sed -r 's/^.* (0x[^. ]*)[. ].*/\1/' "$common_trace_txt_file"
+  else
+    sed -r 's/.*pc=//' "$common_trace_txt_file"
+  fi
+) | \
   xargs "${host_dir}/bin/${arch}-linux-addr2line" -e "${kernel_dir}/vmlinux" -fp | \
   sed -E "s|at ${kernel_dir}/(\./\|)||" | \
   uniq -c \
-> "${qemu_out_dir}/trace-lines.txt"
+> "${common_out_run_dir}/trace-lines.txt"