[lkrn] Add basic support for the RISC-V Linux kernel image format

The RISC-V and AArch64 bare-metal kernel images share a common header
format, and require essentially the same execution environment: loaded
close to the start of RAM, entered with paging disabled, and passed a
pointer to a flattened device tree that describes the hardware and any
boot arguments.

Implement basic support for executing bare-metal RISC-V and AArch64
kernel images.  The (trivial) AArch64-specific code path is untested
since we do not yet have the ability to build for any bare-metal
AArch64 platforms.  Constructing and passing an initramfs image is not
yet supported.

Rename the IMAGE_BZIMAGE build configuration option to IMAGE_LKRN,
since "bzImage" is specific to x86.  To retain backwards compatibility
with existing local build configurations, we leave IMAGE_BZIMAGE as
the enabled option in config/default/pcbios.h and treat IMAGE_LKRN as
a synonym for IMAGE_BZIMAGE when building for x86 BIOS.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/bits/lkrn.h

@@ -0,0 +1,17 @@
+#ifndef _BITS_LKRN_H
+#define _BITS_LKRN_H
+
+/** @file
+ *
+ * Dummy architecture-specific Linux kernel image invocation
+ *
+ * This file is included only if the architecture does not provide its
+ * own version of this file.
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#define LKRN_MAGIC_ARCH 0
+
+#endif /* _BITS_LKRN_H */

src/include/ipxe/errfile.h

@@ -328,6 +328,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define ERRFILE_zlib		      ( ERRFILE_IMAGE | 0x000b0000 )
 #define ERRFILE_gzip		      ( ERRFILE_IMAGE | 0x000c0000 )
 #define ERRFILE_efi_siglist	      ( ERRFILE_IMAGE | 0x000d0000 )
+#define ERRFILE_lkrn		      ( ERRFILE_IMAGE | 0x000e0000 )
 
 #define ERRFILE_asn1		      ( ERRFILE_OTHER | 0x00000000 )
 #define ERRFILE_chap		      ( ERRFILE_OTHER | 0x00010000 )

src/include/ipxe/lkrn.h

@@ -0,0 +1,69 @@
+#ifndef _IPXE_LKRN_H
+#define _IPXE_LKRN_H
+
+/** @file
+ *
+ * Linux kernel images
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <stdint.h>
+
+/** Kernel image header */
+struct lkrn_header {
+	/** Executable code */
+	uint32_t code[2];
+	/** Image load offset */
+	uint64_t text_offset;
+	/** Image size */
+	uint64_t image_size;
+	/** Flags */
+	uint64_t flags;
+	/** Reserved */
+	uint8_t reserved_a[24];
+	/** Magic */
+	uint32_t magic;
+	/** Reserved */
+	uint8_t reserved_b[4];
+} __attribute__ (( packed ));
+
+/** Kernel magic value */
+#define LKRN_MAGIC( a, b, c, d ) \
+	( ( (a) << 0 ) | ( (b) << 8 ) | ( (c) << 16 ) | ( (d) << 24 ) )
+
+/** Kernel magic value for AArch64 */
+#define LKRN_MAGIC_AARCH64 LKRN_MAGIC ( 'A', 'R', 'M', 0x64 )
+
+/** Kernel magic value for RISC-V */
+#define LKRN_MAGIC_RISCV LKRN_MAGIC ( 'R', 'S', 'C', 0x05 )
+
+/** Kernel image context */
+struct lkrn_context {
+	/** Load offset */
+	size_t offset;
+	/** File size */
+	size_t filesz;
+	/** Memory size */
+	size_t memsz;
+
+	/** Start of RAM */
+	physaddr_t ram;
+	/** Entry point */
+	physaddr_t entry;
+	/** Device tree */
+	physaddr_t fdt;
+};
+
+#include <bits/lkrn.h>
+
+/**
+ * Jump to kernel entry point
+ *
+ * @v entry		Kernel entry point
+ * @v fdt		Device tree
+ */
+void lkrn_jump ( physaddr_t entry, physaddr_t fdt );
+
+#endif /* _IPXE_LKRN_H */