Allow the kernel to run as a Linux image

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>

2 files changed, 59 insertions, 3 deletions

diff --git a/kernel/fbos.ld.S b/kernel/fbos.ld.S
index f74d90f..160a5cb 100644
--- a/kernel/fbos.ld.S
+++ b/kernel/fbos.ld.S
@@ -1,8 +1,11 @@
 #include <fbos/mm.h>
 
+OUTPUT_ARCH(riscv)
+ENTRY(_start)
+
 SECTIONS {
 	// Ensure that the image starts at the very exact address SBI expects it to.
-	. = LINK_ADDR;
+	. = LOAD_OFFSET;
 	_start = .;
 	. = ALIGN(PAGE_SIZE);
 
@@ -10,8 +13,9 @@ SECTIONS {
 	// instead of clumping it into the main `.text` one. Well, I'm no expert on
 	// linker configuration, so patches are welcome :)
 	.text : {
+		// head.S
+		KEEP(*(.head.text))
 		_text = .;
-		*(.head.text)
 
 		// Aligning it to a full page is maybe a bit too much considering how
 		// small `.text.head` really is. I just saw this same thing on the Linux
@@ -19,7 +23,7 @@ SECTIONS {
 		. = ALIGN(PAGE_SIZE);
 
 		// From now on the rest of the `.text` could just be a continuation, but
-		// I further split it into `.text.kernel` so the jump from head isn't
+		// I further split it into `.kernel.text` so the jump from head isn't
 		// that large. That is, we put first the very core of the kernel, and
 		// the rest can go wherever.
 		__kernel_text_start = .;
diff --git a/kernel/head.S b/kernel/head.S
index c8f523d..210eb7c 100644
--- a/kernel/head.S
+++ b/kernel/head.S
@@ -1,9 +1,61 @@
+#include <fbos/image.h>
 #include <fbos/mm.h>
 
 .global _start
+.global _start_kernel
 .section .head.text
 
+// Since this kernel boots in virtualization environments and machines that are
+// expecting a Linux kernel, we have to adhere to how bootloaders expect Linux
+// kernel images to look like. In particular, the entry of the kernel is
+// actually a Boot image header, as defined in:
+//
+// 		https://docs.kernel.org/arch/riscv/boot-image-header.html
+//
+// Conveniently, for machines who don't care about this (e.g. QEMU), the first
+// 64 bits correspond to two instructions that can be set up, and hence entering
+// here will simply jumpt to `_start_kernel`, our real entry.
 _start:
+	// The first two words give us room for two executable instructions. Linux
+	// uses that on EFI support to first allocate a magic value for UEFI and
+	// then have a `j _start_kernel` instruction. Otherwise it just allocates
+	// the first one for `j _start_kernel` and leaves the second word empty. The
+	// latter is what we do here as well.
+	j _start_kernel
+	.word 0
+
+	// Ensure alignment for the next double word.
+	.balign 8
+
+	// Load offset. Note that this matches the PAGE_OFFSET as defined in
+	// `include/mm.h`.
+	.dword 0x200000
+
+	// Size of the image. This is *mandatory* as per bootloader request.
+	.dword _end - _start
+
+	// Flags. As defined by Linux, only one bit matters here, which is related
+	// to endianness. Setting 0 means little-endian.
+	.dword 0
+
+	// Header version.
+	.word RISCV_HEADER_VERSION
+
+	// Reserved fields.
+	.word 0
+	.dword 0
+
+	// Deprecated image magic.
+	.ascii RISCV_IMAGE_MAGIC
+	.balign 4
+
+	// Good image magic, in little-endian format.
+	.ascii RISCV_IMAGE_MAGIC2
+
+	// Reserved field.
+	.word 0
+
+_start_kernel:
 	// Mask all interrupts
 	csrw sie, zero
 	csrw sip, zero