build runs, lots of uncommented and lots of untested though

Refactor everything. Create nice submodules/ and packages/ folders.

packages/gem5/.gitignore

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+/system

packages/gem5/Config.in

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+config BR2_PACKAGE_GEM5
+	bool "gem5"
+	help
+	  gem5 system simulator. Only builds the m5 guest instrumentation
+	  tool for now, not the simulator itself.
+
+	  http://gem5.org

packages/gem5/README.adoc

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+https://github.com/cirosantilli/linux-kernel-module-cheat#gem5-directory

packages/gem5/build

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+#!/usr/bin/env bash
+set -eux
+arch=x86_64
+build_type=opt
+cross_compile=
+j=
+outdir="$(pwd)"
+while getopts a:c:j:o:t: OPT; do
+  case "$OPT" in
+    a)
+      arch="$OPTARG"
+      ;;
+    c)
+      cross_compile="CROSS_COMPILE=$OPTARG"
+      ;;
+    j)
+      j="$OPTARG"
+      ;;
+    o)
+      outdir="$OPTARG"
+      ;;
+    t)
+      build_type="$OPTARG"
+      ;;
+    ?)
+      exit 2
+      ;;
+  esac
+done
+shift "$(($OPTIND - 1))"
+if [ -z "$j" ]; then
+  j="$(nproc)"
+fi
+system_dir="${outdir}/system"
+binaries_dir="${system_dir}/binaries"
+disks_dir="${system_dir}/disks"
+mkdir -p "$binaries_dir" "$disks_dir"
+export PATH="/usr/lib/ccache:${PATH}"
+if [ "$arch" = x86_64 ]; then
+  scons -j "$j" --ignore-style "${outdir}/build/X86/gem5.${build_type}"
+  f="${disks_dir}/linux-bigswap2.img"
+  dd if=/dev/zero of="$f" bs=1024 count=65536
+  mkswap "$f"
+  # This file must always be present, despite --kernel overriding that default and selecting the kernel.
+  # I'm not even joking. No one has ever built x86 gem5 without the magic dist dir present.
+  touch "${binaries_dir}/x86_64-vmlinux-2.6.22.9"
+elif [ "$arch" = arm ] || [ "$arch" = aarch64 ]; then
+  scons -j "$j" --ignore-style "${outdir}/build/ARM/gem5.${build_type}"
+  make -C ./system/arm/dt/
+  mkdir -p "${system_dir}/arm/dt"
+  # || true in case they are the same directory.
+  cp ./system/arm/dt/*.dtb "${system_dir}/arm/dt" || true
+  # TODO use the buildroot cross compiler here, and remove the dependencies from configure.
+  make -C ./system/arm/simple_bootloader/ $cross_compile
+  cp ./system/arm/simple_bootloader/boot_emm.arm "$binaries_dir"
+  # TODO cross_compile is ignored because the make does not use CC...
+  make -C ./system/arm/aarch64_bootloader/ $cross_compile
+  cp ./system/arm/aarch64_bootloader/boot_emm.arm64 "$binaries_dir"
+fi
+make -C util/term
+cp util/term/m5term "${outdir}"

packages/gem5/external.desc

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+name: GEM5

packages/gem5/external.mk

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+################################################################################
+#
+# GEM5
+#
+################################################################################
+
+GEM5_VERSION = 1.0
+GEM5_SITE = $(BR2_EXTERNAL_GEM5_PATH)
+GEM5_SITE_METHOD = local
+
+ifeq ($(ARCH),x86_64)
+ARCH_MAKE = x86
+else
+ARCH_MAKE = $(ARCH)
+endif
+
+define GEM5_BUILD_CMDS
+	# TODO cannot pass "-c '$(TARGET_CROSS)'" here because the ARM build uses aarch64 for the bootloader...
+	cd '$(GEM5_LKMC_SRCDIR)' && '$(GEM5_SITE)/build' -a '$(ARCH)' -j '$(BR2_JLEVEL)' -o '$(GEM5_LKMC_OUTDIR)' -t '$(GEM5_LKMC_GEM5_BUILD_TYPE)'
+
+	# TODO cannot use TARGET_CONFIGURE_OPTS here because it overrides the CFLAGS on m5,
+	# which have an include. We should patch gem5 to add a += instead of = there.
+	cd '$(@D)/gem5/util/m5' && $(MAKE) -f 'Makefile.$(ARCH_MAKE)' CC='$(TARGET_CC)' LD='$(TARGET_LD)'
+endef
+
+define GEM5_INSTALL_TARGET_CMDS
+	$(INSTALL) -D -m 0755 '$(@D)/gem5/util/m5/m5' '$(TARGET_DIR)/usr/bin'
+endef
+
+$(eval $(generic-package))