From d51665904faa31c39106e4b2afa7541615072e72 Mon Sep 17 00:00:00 2001
From: Miquel Sabaté Solà <mikisabate@gmail.com>
Date: Fri, 29 Nov 2024 22:17:13 +0100
Subject: Allow the kernel to run as a Linux image
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In order to run on real hardware, it's actually easier to re-use
workflows from existing bootloaders than loading things in memory
manually. In order to achieve this two things had to be settled:

1. The image has to have a Linux header as defined in the RISC-V port.
   This header will be taken into consideration by bootloaders in order to
   know where to jump, the image offset, the endianness of the image, etc.
2. The image cannot just be an ELF executable. Rather, we must translate
   it into binary form via `objcopy`.

With all of this at hand, I was able to make the kernel run on a
Starfive VisionFive 2 board, and I have recorded an example so it's
available as documentation.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>
---
 Makefile | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

(limited to 'Makefile')

diff --git a/Makefile b/Makefile
index 70000a1..5c95c02 100644
--- a/Makefile
+++ b/Makefile
@@ -20,7 +20,8 @@ endif
 CC     = $(CROSS_COMPILE)gcc$(CC_SUFFIX)
 LD     = $(CROSS_COMPILE)ld
 HOSTCC = gcc$(CC_SUFFIX)
-QEMU ?= qemu-system-riscv64
+QEMU  ?= qemu-system-riscv64
+OBJCPY = $(CROSS_COMPILE)objcopy
 
 ISA      ?= rv64imafdc_zicntr_zicsr_zifencei_zihpm_zca_zcd_zba_zbb
 ASFLAGS   = -march=$(ISA) -mabi=lp64d -mcmodel=medany -fno-PIE
@@ -59,7 +60,8 @@ endif
 SRC     = $(filter-out kernel/fbos.ld.S, $(wildcard kernel/*.S kernel/*.c lib/*.c lib/*.S))
 OBJ     = $(patsubst %.c,%.o,$(patsubst %.S,%.o,$(SRC)))
 LINKER  = kernel/fbos.ld
-KRNL    = fbos
+IMAGE   = fbos
+KRNL    = $(IMAGE).elf
 USR     = usr/bin/init usr/bin/fizz usr/bin/buzz usr/bin/fizzbuzz
 INIT    = usr/initramfs.cpio
 TESTS   = test/test_dt test/test_initrd
@@ -71,7 +73,13 @@ LDFLAGS += -T $(LINKER)
 # Kernel
 
 .PHONY: all
-all: clean $(KRNL) usr test
+all: clean $(IMAGE) usr test
+
+.PHONY: $(IMAGE)
+$(IMAGE): $(KRNL)
+	$(E) "	OBJCOPY	" $@
+	$(Q) $(OBJCPY) $(KRNL) -O binary $(IMAGE)
+	$(Q) rm $(KRNL)
 
 .PHONY: $(KRNL)
 $(KRNL): $(OBJ) $(LINKER).S
@@ -134,7 +142,7 @@ test/%: test/%.o
 .PHONY: archive
 archive: all
 	$(Q) $(eval DIR := $(shell mktemp -d))
-	$(Q) mkdir -p $(DIR)/fbos && cp $(KRNL) $(INIT) $(DIR)/fbos/
+	$(Q) mkdir -p $(DIR)/fbos && cp $(IMAGE) $(INIT) $(DIR)/fbos/
 	$(Q) cd $(DIR) && tar czf $(ARCHIVE) fbos/
 	$(Q) cp $(DIR)/$(ARCHIVE) .
 	$(E) "	TAR	 $(ARCHIVE)"
@@ -143,11 +151,11 @@ archive: all
 # Hacking
 
 .PHONY: qemu
-qemu: clean $(KRNL) usr
+qemu: clean $(IMAGE) usr
 ifeq ($(strip $(QEMU_BIOS)),)
-	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -kernel $(KRNL) -initrd $(INIT)
+	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -kernel $(IMAGE) -initrd $(INIT)
 else
-	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -bios $(QEMU_BIOS) -kernel $(KRNL) -initrd $(INIT)
+	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -bios $(QEMU_BIOS) -kernel $(IMAGE) -initrd $(INIT)
 endif
 
 .PHONY: gdb
@@ -156,7 +164,7 @@ gdb:
 
 .PHONY: clean
 clean:
-	$(Q) rm -rf $(OBJ) $(KRNL) $(LINKER) $(USR) usr/src/*.o $(INIT) test/*.o test/lib/*.o $(TESTS) $(ARCHIVE)
+	$(Q) rm -rf $(OBJ) $(KRNL) $(IMAGE) $(LINKER) $(USR) usr/src/*.o $(INIT) test/*.o test/lib/*.o $(TESTS) $(ARCHIVE)
 
 .PHONY: lint
 lint:
-- 
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