1 files changed, 52 insertions, 0 deletions
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