0001 =================================
0002 Boot image header in RISC-V Linux
0003 =================================
0004
0005 :Author: Atish Patra <atish.patra@wdc.com>
0006 :Date: 20 May 2019
0007
0008 This document only describes the boot image header details for RISC-V Linux.
0009
0010 TODO:
0011 Write a complete booting guide.
0012
0013 The following 64-byte header is present in decompressed Linux kernel image::
0014
0015 u32 code0; /* Executable code */
0016 u32 code1; /* Executable code */
0017 u64 text_offset; /* Image load offset, little endian */
0018 u64 image_size; /* Effective Image size, little endian */
0019 u64 flags; /* kernel flags, little endian */
0020 u32 version; /* Version of this header */
0021 u32 res1 = 0; /* Reserved */
0022 u64 res2 = 0; /* Reserved */
0023 u64 magic = 0x5643534952; /* Magic number, little endian, "RISCV" */
0024 u32 magic2 = 0x05435352; /* Magic number 2, little endian, "RSC\x05" */
0025 u32 res3; /* Reserved for PE COFF offset */
0026
0027 This header format is compliant with PE/COFF header and largely inspired from
0028 ARM64 header. Thus, both ARM64 & RISC-V header can be combined into one common
0029 header in future.
0030
0031 Notes
0032 =====
0033
0034 - This header can also be reused to support EFI stub for RISC-V in future. EFI
0035 specification needs PE/COFF image header in the beginning of the kernel image
0036 in order to load it as an EFI application. In order to support EFI stub,
0037 code0 should be replaced with "MZ" magic string and res3(at offset 0x3c) should
0038 point to the rest of the PE/COFF header.
0039
0040 - version field indicate header version number
0041
0042 ========== =============
0043 Bits 0:15 Minor version
0044 Bits 16:31 Major version
0045 ========== =============
0046
0047 This preserves compatibility across newer and older version of the header.
0048 The current version is defined as 0.2.
0049
0050 - The "magic" field is deprecated as of version 0.2. In a future
0051 release, it may be removed. This originally should have matched up
0052 with the ARM64 header "magic" field, but unfortunately does not.
0053 The "magic2" field replaces it, matching up with the ARM64 header.
0054
0055 - In current header, the flags field has only one field.
0056
0057 ===== ====================================
0058 Bit 0 Kernel endianness. 1 if BE, 0 if LE.
0059 ===== ====================================
0060
0061 - Image size is mandatory for boot loader to load kernel image. Booting will
0062 fail otherwise.
