Document gem5 userland debugging

This lead to an infinite chaing of refactorings:

Fix arm and aarch64 lx-symbols

This was just by chance, a continue GDB command was needed before running it.

I noticed that there is no more need for the x86 GDB --ex 'set arch i386:x86-64:intel'
thing and unified all archs. TODO bisect where that was fixed and understand why.

Add debug kernel configs for gem5 kernel.

Document gem5 aarch64 gdb debug doesn't work.

Document tmux split pane action.

run

@@ -6,6 +6,7 @@ set -e
 arch=x86_64
 cpus=1
 debug_vm=''
+debug=false
 kgdb=false
 kvm=false
 # norandmaps: Don't use address space randomization. Equivalent to echo 0 > /proc/sys/kernel/randomize_va_space.
@@ -36,7 +37,8 @@ while getopts a:c:DdE:e:f:G:gIiKkm:nt:x OPT; do
       debug_vm='gdb -q -ex start --args'
       ;;
     d)
-      extra_flags_qemu="$extra_flags_qemu -S -s"
+      debug=true
+      extra_flags_qemu="$extra_flags_qemu -S"
       ;;
     E)
       lkmc_eval="$OPTARG"
@@ -84,6 +86,10 @@ done
 shift "$(($OPTIND - 1))"
 extra_flags="$extra_flags $@"
 set_common_vars "$arch" "$gem5"
+if "$debug" && "$kvm"; then
+  echo 'error: -d and -K are incompatible' 1>&2
+  exit 1
+fi
 if "$initrd" || "$initramfs"; then
   ramfs=true
 else
@@ -159,10 +165,11 @@ else
   qemu_common="\
 ${debug_vm} \
 '${out_dir}/host/usr/bin/qemu-system-${arch}' \
+-gdb tcp::1234 \
 -m ${memory} \
 -monitor telnet::45454,server,nowait \
 -netdev user,hostfwd=tcp::45455-:45455,id=net0 \
--smp $cpus \
+-smp ${cpus} \
 -virtfs local,path=9p,mount_tag=host_scratch,security_model=mapped,id=host_scratch \
 -virtfs local,path=${out_dir}/build,mount_tag=host_out,security_model=mapped,id=host_out \
 "