diff --git a/build b/build
index bef5ae79..7fb5b879 100755
--- a/build
+++ b/build
@@ -20,7 +20,7 @@ post_script_args=
 qemu_sdl='--enable-sdl --with-sdlabi=2.0'
 suffix=
 v=0
-while getopts 'a:B:b:C:c:fGgj:hIiK:kL:lp:qSs:v' OPT; do
+while getopts 'a:B:b:C:c:fGgj:hIiK:kL:lM:p:qSs:v' OPT; do
   case "$OPT" in
     a)
       arch="$OPTARG"
@@ -81,6 +81,9 @@ BR2_TARGET_ROOTFS_INITRAMFS=n
       extra_make_args="${extra_make_args} linux-reconfigure \\
 "
       ;;
+    M)
+      common_gem5_variant="$OPTARG"
+      ;;
     p)
       post_script_args="$OPTARG"
       ;;
@@ -104,7 +107,7 @@ BR2_TARGET_ROOTFS_INITRAMFS=n
 done
 shift $(($OPTIND - 1))
 extra_make_args="${extra_make_args} $@"
-set_common_vars -L "$common_linux_variant" "$arch" "$gem5" "$suffix"
+set_common_vars -L "$common_linux_variant" -M "$common_gem5_variant" "$arch" "$gem5" "$suffix"
 config_file="${buildroot_out_dir}/.config"
 case "$arch" in
   x86_64)
@@ -170,6 +173,8 @@ BR2_ROOTFS_POST_SCRIPT_ARGS=\"${post_script_args}\"
 fi
 
 common_mkdir
+
+# Manage Linux kernel variants.
 if [ -h "$common_linux_custom_dir" ]; then
   rm "$common_linux_custom_dir"
 elif [ -d "$common_linux_custom_dir" ]; then
@@ -178,6 +183,16 @@ elif [ -d "$common_linux_custom_dir" ]; then
 fi
 mkdir -p "$common_linux_variant_dir"
 ln -s "$common_linux_variant_dir" "$common_linux_custom_dir"
+
+# Manage gem5 variants.
+if "$gem5"; then
+  if [ -e "${gem5_out_dir}/.git" ]; then
+    git -C "$gem5_out_dir" checkout "$(git -C "$common_gem5_src_dir" log -n 1 --format='%H')"
+  else
+    git -C "$common_gem5_src_dir" worktree add -b "${common_gem5_variant}" "${gem5_out_dir}"
+  fi
+fi
+
 cd "$buildroot_dir"
 # HOST_QEMU_OPTS is a hack that happens to work because the QEMU package luckly uses += at all times.
 # It shouldn't be necessary in the first place: https://bugs.busybox.net/show_bug.cgi?id=9936
@@ -193,6 +208,7 @@ env \\
 make \\
 O='${buildroot_out_dir}' \\
 HOST_QEMU_OPTS='--enable-debug --enable-trace-backends=simple ${qemu_sdl}' \\
+GEM5_LKMC_OUTDIR="$gem5_out_dir" \\
 V='${v}' \\
 ${extra_make_args} \
 all \\
diff --git a/common b/common
index ab653948..ebf75abd 100644
--- a/common
+++ b/common
@@ -21,12 +21,16 @@ common_bench_cmd() (
 )
 set_common_vars() {
   linux_variant=
+  gem5_variant=
   OPTIND=1
-  while getopts L: OPT; do
+  while getopts L:M: OPT; do
     case "$OPT" in
       L)
         linux_variant="$OPTARG"
         ;;
+      M)
+        gem5_variant="$OPTARG"
+        ;;
     esac
   done
   echo $OPTIND
@@ -75,6 +79,7 @@ set_common_vars() {
   common_linux_custom_dir="${build_dir}/linux-custom"
   common_linux_variant_dir="${common_linux_custom_dir}.${linux_variant}"
   common_vmlinux="${common_linux_variant_dir}/vmlinux"
+  gem5_out_dir="${common_dir}/gem5.${gem5_variant}"
   case "$arch" in
     arm)
       common_linux_image=arch/arm/boot/zImage
@@ -100,6 +105,7 @@ common_mkdir() (
   ;
 )
 common_linux_variant=default
+common_gem5_variant=default
 root_dir="$(pwd)"
 out_dir="${root_dir}/out"
 common_bench_boot="${out_dir}/bench-boot.txt"
@@ -107,7 +113,8 @@ data_dir="${root_dir}/data"
 p9_dir="${data_dir}/9p"
 readfile_file="${data_dir}/readfile"
 common_dir="${out_dir}/common"
-gem5_out_dir="${common_dir}/gem5"
+gem5_variant=default
+common_gem5_src_dir="${root_dir}/gem5/gem5"
 f="${data_dir}/cli"
 if [ -f "$f" ]; then
   . "$f"
diff --git a/gem5/build b/gem5/build
index f5ee553f..a107342a 100755
--- a/gem5/build
+++ b/gem5/build
@@ -3,7 +3,7 @@ set -eu
 arch=x86_64
 cross_compile=
 j=
-outdir="$(pwd)/out"
+outdir="$(pwd)"
 while getopts a:c:j:o: OPT; do
   case "$OPT" in
     a)
@@ -28,7 +28,6 @@ system_dir="${outdir}/system"
 binaries_dir="${system_dir}/binaries"
 disks_dir="${system_dir}/disks"
 mkdir -p "$binaries_dir" "$disks_dir"
-cd "gem5"
 if [ "$arch" = x86_64 ]; then
   scons -j "$j" --ignore-style "${outdir}/build/X86/gem5.opt"
   f="${disks_dir}/linux-bigswap2.img"
diff --git a/gem5/external.mk b/gem5/external.mk
index 94f002ad..d5ee1cd0 100644
--- a/gem5/external.mk
+++ b/gem5/external.mk
@@ -16,7 +16,13 @@ endif
 
 define GEM5_BUILD_CMDS
 	# Cannot pass "-c '$(TARGET_CROSS)'" here because the ARM build uses aarch64 for the bootloader...
-	cd '$(GEM5_SITE)' && ./build -a '$(ARCH)' -j '$(BR2_JLEVEL)' -o '$(GEM5_SITE)/../out/common/gem5'
+	cd '$(GEM5_LKMC_OUTDIR)' && '$(GEM5_SITE)/build' -a '$(ARCH)' -j '$(BR2_JLEVEL)'
+
+	# This would build using the submodule source out of tree.
+	# We gave up on it because in the end we still need the python scripts to match:
+	# we need to have an entire copy of the tree somwhere to switch between them.
+	#cd '$(GEM5_SITE)/gem5' && ../build -a '$(ARCH)' -j '$(BR2_JLEVEL)' -o '$(GEM5_SITE)/../out/common/gem5'
+
 	# This would build inside the buildroot directory as a more normal package.
 	#cd '$(@D)' && ./build -a '$(ARCH)' -j '$(BR2_JLEVEL)'
 
diff --git a/run b/run
index d719c060..a2955fac 100755
--- a/run
+++ b/run
@@ -34,7 +34,7 @@ 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:PT:U:uVx OPT; do
+while getopts a:c:DdE:e:F:f:G:ghIiKkL:M:m:PT:U:uVx OPT; do
   case "$OPT" in
     a)
       arch="$OPTARG"
@@ -94,6 +94,9 @@ while getopts a:c:DdE:e:F:f:G:ghIiKkL:m:PT:U:uVx OPT; do
     L)
       common_linux_variant="$OPTARG"
       ;;
+    M)
+      common_gem5_variant="$OPTARG"
+      ;;
     m)
       memory="$OPTARG"
       ;;
@@ -124,7 +127,7 @@ while getopts a:c:DdE:e:F:f:G:ghIiKkL:m:PT:U:uVx OPT; do
 done
 shift "$(($OPTIND - 1))"
 extra_flags="$extra_flags $@"
-set_common_vars -L "$common_linux_variant" "$arch" "$gem5"
+set_common_vars -L "$common_linux_variant" -M "$common_gem5_variant" "$arch" "$gem5"
 images_dir="${buildroot_out_dir}/images"
 if "$debug" && "$kvm"; then
   echo 'error: -d and -K are incompatible' 1>&2
@@ -156,8 +159,6 @@ if [ -n "$extra_append_after_dash" ]; then
 fi
 
 if "$gem5"; then
-  gem5_build_dir="${buildroot_out_dir}/build/gem5-1.0"
-  gem5_src_dir="${root_dir}/gem5/gem5"
   gem5_system_dir="${gem5_out_dir}/system"
   memory="${memory}B"
   if [ "$arch" = x86_64 ]; then
@@ -176,7 +177,7 @@ ${debug_vm} \
 --debug-file=trace.txt \\
 ${gem5opts} \
 -d '${m5out_dir}' \\
-'${gem5_src_dir}/configs/example/fs.py' \\
+'${gem5_out_dir}/configs/example/fs.py' \\
 --disk-image='${images_dir}/rootfs.ext2' \\
 --kernel='${common_vmlinux}' \\
 --mem-size='${memory}' \\