feat: root: add dummy root server

Makefile

@@ -31,7 +31,7 @@ QEMU = qemu-system-$(ARCH)
 QEMU_ARGS  = -nographic -serial mon:stdio -smp 1
 QEMU_ARGS += -machine virt # TODO: override by $(BOARD)
 QEMU_ARGS += -kernel target/$(BUILD_TARGET)/$(MODE)/kernel
-QEMU_ARGS += -initrd build/init.cpio
+QEMU_ARGS += -initrd root/init.cpio
 QEMU_ARGS += -append "loglevel=4"
 
 kernel:

root/.gitignore

@@ -0,0 +1,3 @@
+*.cpio
+*.o
+root

root/Makefile

@@ -0,0 +1,21 @@
+AS = riscv64-elf-as
+LD = riscv64-elf-ld
+
+ASFLAGS += -march=rv64imac -mabi=lp64
+
+.PHONY: all clean
+
+all: init.cpio
+
+%.o: %.S
+	$(AS) $(ASFLAGS) -c $< -o $@
+
+root: root.o
+	$(LD) -Tlink.ld $^ -o $@
+
+init.cpio: root
+	find . -type f -not -name "*.cpio" | cpio -ov -H crc > init.cpio
+
+clean:
+	rm -f *.o root
+	rm -f init.cpio

root/link.ld

@@ -0,0 +1,16 @@
+OUTPUT_ARCH(riscv)
+ENTRY(_start)
+
+PHDRS {
+    text   PT_LOAD FLAGS (5);
+    rodata PT_LOAD FLAGS (4);
+    data   PT_LOAD FLAGS (6);
+}
+
+SECTIONS {
+    . = 0x400000;
+    .text   : { *(.text   .text.*  ); . = ALIGN(4096); } : text
+    .rodata : { *(.rodata .rodata.*); . = ALIGN(4096); } : rodata
+    .data   : { *(.data   .data.*  ); . = ALIGN(4096); } : data
+    .bss    : { *(.bss    .bss.*   ); . = ALIGN(4096); } : data
+}

root/root.S

@@ -0,0 +1,19 @@
+.macro syscall msg ptr
+	li	a0, \msg
+	li	a1, \ptr
+	ecall
+.endm
+
+.text
+	.global _start
+_start:
+	la	t0, msg
+.loop:
+	lb	a2, 0(t0)
+	beqz	a2, _start	# print forever
+	syscall (1<<12 | 1), 1	# see uapi/syscall.rs
+	addi	t0, t0, 1
+	j	.loop
+
+.data
+	msg : .string "Hello, world!\n\0"