selftests/nolibc: don't embed initramfs into kernel image

When the initramfs is embedded into the kernel each rebuild of it will
trigger a full kernel relink and all the expensive postprocessing steps.

Currently nolibc-test and therefore the initramfs are always rebuild,
even without source changes, leading to lots of slow kernel relinks.

Instead of linking the initramfs into the kernel assemble it manually
and pass it explicitly to qemu.
This avoids all of the kernel relinks.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Link: https://lore.kernel.org/r/20230917-nolibc-initramfs-v2-1-f0f293a8b198@weissschuh.net

tools/testing/selftests/nolibc/.gitignore

 /initramfs/
+/initramfs.cpio
 /libc-test
 /nolibc-test
 /run.out

tools/testing/selftests/nolibc/Makefile

@@ -131,18 +131,20 @@ REPORT  ?= awk '/\[OK\][\r]*$$/{p++} /\[FAIL\][\r]*$$/{if (!f) printf("\n"); f++
 
 help:
 	@echo "Supported targets under selftests/nolibc:"
-	@echo "  all          call the \"run\" target below"
-	@echo "  help         this help"
-	@echo "  sysroot      create the nolibc sysroot here (uses \$$ARCH)"
-	@echo "  nolibc-test  build the executable (uses \$$CC and \$$CROSS_COMPILE)"
-	@echo "  libc-test    build an executable using the compiler's default libc instead"
-	@echo "  run-user     runs the executable under QEMU (uses \$$XARCH, \$$TEST)"
-	@echo "  initramfs    prepare the initramfs with nolibc-test"
-	@echo "  defconfig    create a fresh new default config (uses \$$XARCH)"
-	@echo "  kernel       (re)build the kernel with the initramfs (uses \$$XARCH)"
-	@echo "  run          runs the kernel in QEMU after building it (uses \$$XARCH, \$$TEST)"
-	@echo "  rerun        runs a previously prebuilt kernel in QEMU (uses \$$XARCH, \$$TEST)"
-	@echo "  clean        clean the sysroot, initramfs, build and output files"
+	@echo "  all               call the \"run\" target below"
+	@echo "  help              this help"
+	@echo "  sysroot           create the nolibc sysroot here (uses \$$ARCH)"
+	@echo "  nolibc-test       build the executable (uses \$$CC and \$$CROSS_COMPILE)"
+	@echo "  libc-test         build an executable using the compiler's default libc instead"
+	@echo "  run-user          runs the executable under QEMU (uses \$$XARCH, \$$TEST)"
+	@echo "  initramfs.cpio    prepare the initramfs archive with nolibc-test"
+	@echo "  initramfs         prepare the initramfs tree with nolibc-test"
+	@echo "  defconfig         create a fresh new default config (uses \$$XARCH)"
+	@echo "  kernel            (re)build the kernel (uses \$$XARCH)"
+	@echo "  kernel-standalone (re)build the kernel with the initramfs (uses \$$XARCH)"
+	@echo "  run               runs the kernel in QEMU after building it (uses \$$XARCH, \$$TEST)"
+	@echo "  rerun             runs a previously prebuilt kernel in QEMU (uses \$$XARCH, \$$TEST)"
+	@echo "  clean             clean the sysroot, initramfs, build and output files"
 	@echo ""
 	@echo "The output file is \"run.out\". Test ranges may be passed using \$$TEST."
 	@echo ""
@@ -195,6 +197,9 @@ run-user: nolibc-test
 	$(Q)qemu-$(QEMU_ARCH) ./nolibc-test > "$(CURDIR)/run.out" || :
 	$(Q)$(REPORT) $(CURDIR)/run.out
 
+initramfs.cpio: kernel nolibc-test
+	$(QUIET_GEN)echo 'file /init nolibc-test 755 0 0' | $(srctree)/usr/gen_init_cpio - > initramfs.cpio
+
 initramfs: nolibc-test
 	$(QUIET_MKDIR)mkdir -p initramfs
 	$(call QUIET_INSTALL, initramfs/init)
@@ -203,17 +208,20 @@ initramfs: nolibc-test
 defconfig:
 	$(Q)$(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE) mrproper $(DEFCONFIG) prepare
 
-kernel: initramfs
+kernel:
+	$(Q)$(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE) $(IMAGE_NAME)
+
+kernel-standalone: initramfs
 	$(Q)$(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE) $(IMAGE_NAME) CONFIG_INITRAMFS_SOURCE=$(CURDIR)/initramfs
 
 # run the tests after building the kernel
-run: kernel
-	$(Q)qemu-system-$(QEMU_ARCH) -display none -no-reboot -kernel "$(srctree)/$(IMAGE)" -serial stdio $(QEMU_ARGS) > "$(CURDIR)/run.out"
+run: kernel initramfs.cpio
+	$(Q)qemu-system-$(QEMU_ARCH) -display none -no-reboot -kernel "$(srctree)/$(IMAGE)" -initrd initramfs.cpio -serial stdio $(QEMU_ARGS) > "$(CURDIR)/run.out"
 	$(Q)$(REPORT) $(CURDIR)/run.out
 
 # re-run the tests from an existing kernel
 rerun:
-	$(Q)qemu-system-$(QEMU_ARCH) -display none -no-reboot -kernel "$(srctree)/$(IMAGE)" -serial stdio $(QEMU_ARGS) > "$(CURDIR)/run.out"
+	$(Q)qemu-system-$(QEMU_ARCH) -display none -no-reboot -kernel "$(srctree)/$(IMAGE)" -initrd initramfs.cpio -serial stdio $(QEMU_ARGS) > "$(CURDIR)/run.out"
 	$(Q)$(REPORT) $(CURDIR)/run.out
 
 # report with existing test log
@@ -227,6 +235,8 @@ clean:
 	$(Q)rm -f nolibc-test
 	$(call QUIET_CLEAN, libc-test)
 	$(Q)rm -f libc-test
+	$(call QUIET_CLEAN, initramfs.cpio)
+	$(Q)rm -rf initramfs.cpio
 	$(call QUIET_CLEAN, initramfs)
 	$(Q)rm -rf initramfs
 	$(call QUIET_CLEAN, run.out)