riscv: modify the Image header to improve compatibility with the ARM64 header

Part of the intention during the definition of the RISC-V kernel image
header was to lay the groundwork for a future merge with the ARM64
image header.  One error during my original review was not noticing
that the RISC-V header's "magic" field was at a different size and
position than the ARM64's "magic" field.  If the existing ARM64 Image
header parsing code were to attempt to parse an existing RISC-V kernel
image header format, it would see a magic number 0.  This is
undesirable, since it's our intention to align as closely as possible
with the ARM64 header format.  Another problem was that the original
"res3" field was not being initialized correctly to zero.

Address these issues by creating a 32-bit "magic2" field in the RISC-V
header which matches the ARM64 "magic" field.  RISC-V binaries will
store "RSC\x05" in this field.  The intention is that the use of the
existing 64-bit "magic" field in the RISC-V header will be deprecated
over time.  Increment the minor version number of the file format to
indicate this change, and update the documentation accordingly.  Fix
the assembler directives in head.S to ensure that reserved fields are
properly zero-initialized.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Reported-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Cc: Atish Patra <atish.patra@wdc.com>
Cc: Karsten Merker <merker@debian.org>
Link: https://lore.kernel.org/linux-riscv/194c2f10c9806720623430dbf0cc59a965e50448.camel@wdc.com/T/#u
Link: https://lore.kernel.org/linux-riscv/mhng-755b14c4-8f35-4079-a7ff-e421fd1b02bc@palmer-si-x1e/T/#t

Documentation/riscv/boot-image-header.txt

@@ -18,7 +18,7 @@ The following 64-byte header is present in decompressed Linux kernel image.
 	u32 res1  = 0;		  /* Reserved */
 	u64 res2  = 0;    	  /* Reserved */
 	u64 magic = 0x5643534952; /* Magic number, little endian, "RISCV" */
-	u32 res3;		  /* Reserved for additional RISC-V specific header */
+	u32 magic2 = 0x56534905;  /* Magic number 2, little endian, "RSC\x05" */
 	u32 res4;		  /* Reserved for PE COFF offset */
 
 This header format is compliant with PE/COFF header and largely inspired from
@@ -37,13 +37,14 @@ Notes:
 	Bits 16:31 - Major version
 
   This preserves compatibility across newer and older version of the header.
-  The current version is defined as 0.1.
+  The current version is defined as 0.2.
 
-- res3 is reserved for offset to any other additional fields. This makes the
-  header extendible in future. One example would be to accommodate ISA
-  extension for RISC-V in future. For current version, it is set to be zero.
+- The "magic" field is deprecated as of version 0.2.  In a future
+  release, it may be removed.  This originally should have matched up
+  with the ARM64 header "magic" field, but unfortunately does not.
+  The "magic2" field replaces it, matching up with the ARM64 header.
 
-- In current header, the flag field has only one field.
+- In current header, the flags field has only one field.
 	Bit 0: Kernel endianness. 1 if BE, 0 if LE.
 
 - Image size is mandatory for boot loader to load kernel image. Booting will

arch/riscv/include/asm/image.h

@@ -3,7 +3,8 @@
 #ifndef __ASM_IMAGE_H
 #define __ASM_IMAGE_H
 
-#define RISCV_IMAGE_MAGIC	"RISCV"
+#define RISCV_IMAGE_MAGIC	"RISCV\0\0\0"
+#define RISCV_IMAGE_MAGIC2	"RSC\x05"
 
 #define RISCV_IMAGE_FLAG_BE_SHIFT	0
 #define RISCV_IMAGE_FLAG_BE_MASK	0x1
@@ -23,7 +24,7 @@
 #define __HEAD_FLAGS		(__HEAD_FLAG(BE))
 
 #define RISCV_HEADER_VERSION_MAJOR 0
-#define RISCV_HEADER_VERSION_MINOR 1
+#define RISCV_HEADER_VERSION_MINOR 2
 
 #define RISCV_HEADER_VERSION (RISCV_HEADER_VERSION_MAJOR << 16 | \
 			      RISCV_HEADER_VERSION_MINOR)
@@ -39,9 +40,8 @@
  * @version:		version
  * @res1:		reserved
  * @res2:		reserved
- * @magic:		Magic number
- * @res3:		reserved (will be used for additional RISC-V specific
- *			header)
+ * @magic:		Magic number (RISC-V specific; deprecated)
+ * @magic2:		Magic number 2 (to match the ARM64 'magic' field pos)
  * @res4:		reserved (will be used for PE COFF offset)
  *
  * The intention is for this header format to be shared between multiple
@@ -58,7 +58,7 @@ struct riscv_image_header {
 	u32 res1;
 	u64 res2;
 	u64 magic;
-	u32 res3;
+	u32 magic2;
 	u32 res4;
 };
 #endif /* __ASSEMBLY__ */

arch/riscv/kernel/head.S

@@ -39,9 +39,9 @@ ENTRY(_start)
 	.word RISCV_HEADER_VERSION
 	.word 0
 	.dword 0
-	.asciz RISCV_IMAGE_MAGIC
-	.word 0
+	.ascii RISCV_IMAGE_MAGIC
 	.balign 4
+	.ascii RISCV_IMAGE_MAGIC2
 	.word 0
 
 .global _start_kernel