51 lines
2.1 KiB
Plaintext
51 lines
2.1 KiB
Plaintext
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Boot image header in RISC-V Linux
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=============================================
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Author: Atish Patra <atish.patra@wdc.com>
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Date : 20 May 2019
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This document only describes the boot image header details for RISC-V Linux.
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The complete booting guide will be available at Documentation/riscv/booting.txt.
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The following 64-byte header is present in decompressed Linux kernel image.
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u32 code0; /* Executable code */
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u32 code1; /* Executable code */
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u64 text_offset; /* Image load offset, little endian */
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u64 image_size; /* Effective Image size, little endian */
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u64 flags; /* kernel flags, little endian */
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u32 version; /* Version of this header */
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u32 res1 = 0; /* Reserved */
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u64 res2 = 0; /* Reserved */
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u64 magic = 0x5643534952; /* Magic number, little endian, "RISCV" */
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u32 res3; /* Reserved for additional RISC-V specific header */
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u32 res4; /* Reserved for PE COFF offset */
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This header format is compliant with PE/COFF header and largely inspired from
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ARM64 header. Thus, both ARM64 & RISC-V header can be combined into one common
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header in future.
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Notes:
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- This header can also be reused to support EFI stub for RISC-V in future. EFI
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specification needs PE/COFF image header in the beginning of the kernel image
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in order to load it as an EFI application. In order to support EFI stub,
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code0 should be replaced with "MZ" magic string and res5(at offset 0x3c) should
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point to the rest of the PE/COFF header.
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- version field indicate header version number.
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Bits 0:15 - Minor version
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Bits 16:31 - Major version
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This preserves compatibility across newer and older version of the header.
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The current version is defined as 0.1.
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- res3 is reserved for offset to any other additional fields. This makes the
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header extendible in future. One example would be to accommodate ISA
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extension for RISC-V in future. For current version, it is set to be zero.
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- In current header, the flag field has only one field.
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Bit 0: Kernel endianness. 1 if BE, 0 if LE.
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- Image size is mandatory for boot loader to load kernel image. Booting will
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fail otherwise.
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