1Booting Linux on x86 with FIT 2============================= 3 4Background 5---------- 6 7(corrections to the text below are welcome) 8 9Generally Linux x86 uses its own very complex booting method. There is a setup 10binary which contains all sorts of parameters and a compressed self-extracting 11binary for the kernel itself, often with a small built-in serial driver to 12display decompression progress. 13 14The x86 CPU has various processor modes. I am no expert on these, but my 15understanding is that an x86 CPU (even a really new one) starts up in a 16-bit 16'real' mode where only 1MB of memory is visible, moves to 32-bit 'protected' 17mode where 4GB is visible (or more with special memory access techniques) and 18then to 64-bit 'long' mode if 64-bit execution is required. 19 20Partly the self-extracting nature of Linux was introduced to cope with boot 21loaders that were barely capable of loading anything. Even changing to 32-bit 22mode was something of a challenge, so putting this logic in the kernel seemed 23to make sense. 24 25Bit by bit more and more logic has been added to this post-boot pre-Linux 26wrapper: 27 28- Changing to 32-bit mode 29- Decompression 30- Serial output (with drivers for various chips) 31- Load address randomisation 32- Elf loader complete with relocation (for the above) 33- Random number generator via 3 methods (again for the above) 34- Some sort of EFI mini-loader (1000+ glorious lines of code) 35- Locating and tacking on a device tree and ramdisk 36 37To my mind, if you sit back and look at things from first principles, this 38doesn't make a huge amount of sense. Any boot loader worth its salts already 39has most of the above features and more besides. The boot loader already knows 40the layout of memory, has a serial driver, can decompress things, includes an 41ELF loader and supports device tree and ramdisks. The decision to duplicate 42all these features in a Linux wrapper caters for the lowest common 43denominator: a boot loader which consists of a BIOS call to load something off 44disk, followed by a jmp instruction. 45 46(Aside: On ARM systems, we worry that the boot loader won't know where to load 47the kernel. It might be easier to just provide that information in the image, 48or in the boot loader rather than adding a self-relocator to put it in the 49right place. Or just use ELF? 50 51As a result, the x86 kernel boot process is needlessly complex. The file 52format is also complex, and obfuscates the contents to a degree that it is 53quite a challenge to extract anything from it. This bzImage format has become 54so prevalent that is actually isn't possible to produce the 'raw' kernel build 55outputs with the standard Makefile (as it is on ARM for example, at least at 56the time of writing). 57 58This document describes an alternative boot process which uses simple raw 59images which are loaded into the right place by the boot loader and then 60executed. 61 62 63Build the kernel 64---------------- 65 66Note: these instructions assume a 32-bit kernel. U-Boot also supports directly 67booting a 64-bit kernel by jumping into 64-bit mode first (see below). 68 69You can build the kernel as normal with 'make'. This will create a file called 70'vmlinux'. This is a standard ELF file and you can look at it if you like: 71 72$ objdump -h vmlinux 73 74vmlinux: file format elf32-i386 75 76Sections: 77Idx Name Size VMA LMA File off Algn 78 0 .text 00416850 81000000 01000000 00001000 2**5 79 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE 80 1 .notes 00000024 81416850 01416850 00417850 2**2 81 CONTENTS, ALLOC, LOAD, READONLY, CODE 82 2 __ex_table 00000c50 81416880 01416880 00417880 2**3 83 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 84 3 .rodata 00154b9e 81418000 01418000 00419000 2**5 85 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 86 4 __bug_table 0000597c 8156cba0 0156cba0 0056dba0 2**0 87 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 88 5 .pci_fixup 00001b80 8157251c 0157251c 0057351c 2**2 89 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 90 6 .tracedata 00000024 8157409c 0157409c 0057509c 2**0 91 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 92 7 __ksymtab 00007ec0 815740c0 015740c0 005750c0 2**2 93 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 94 8 __ksymtab_gpl 00004a28 8157bf80 0157bf80 0057cf80 2**2 95 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 96 9 __ksymtab_strings 0001d6fc 815809a8 015809a8 005819a8 2**0 97 CONTENTS, ALLOC, LOAD, READONLY, DATA 98 10 __init_rodata 00001c3c 8159e0a4 0159e0a4 0059f0a4 2**2 99 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 100 11 __param 00000ff0 8159fce0 0159fce0 005a0ce0 2**2 101 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 102 12 __modver 00000330 815a0cd0 015a0cd0 005a1cd0 2**2 103 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 104 13 .data 00063000 815a1000 015a1000 005a2000 2**12 105 CONTENTS, ALLOC, LOAD, RELOC, DATA 106 14 .init.text 0002f104 81604000 01604000 00605000 2**2 107 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE 108 15 .init.data 00040cdc 81634000 01634000 00635000 2**12 109 CONTENTS, ALLOC, LOAD, RELOC, DATA 110 16 .x86_cpu_dev.init 0000001c 81674cdc 01674cdc 00675cdc 2**2 111 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 112 17 .altinstructions 0000267c 81674cf8 01674cf8 00675cf8 2**0 113 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 114 18 .altinstr_replacement 00000942 81677374 01677374 00678374 2**0 115 CONTENTS, ALLOC, LOAD, READONLY, CODE 116 19 .iommu_table 00000014 81677cb8 01677cb8 00678cb8 2**2 117 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 118 20 .apicdrivers 00000004 81677cd0 01677cd0 00678cd0 2**2 119 CONTENTS, ALLOC, LOAD, RELOC, DATA 120 21 .exit.text 00001a80 81677cd8 01677cd8 00678cd8 2**0 121 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE 122 22 .data..percpu 00007880 8167a000 0167a000 0067b000 2**12 123 CONTENTS, ALLOC, LOAD, RELOC, DATA 124 23 .smp_locks 00003000 81682000 01682000 00683000 2**2 125 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA 126 24 .bss 000a1000 81685000 01685000 00686000 2**12 127 ALLOC 128 25 .brk 00424000 81726000 01726000 00686000 2**0 129 ALLOC 130 26 .comment 00000049 00000000 00000000 00686000 2**0 131 CONTENTS, READONLY 132 27 .GCC.command.line 0003e055 00000000 00000000 00686049 2**0 133 CONTENTS, READONLY 134 28 .debug_aranges 0000f4c8 00000000 00000000 006c40a0 2**3 135 CONTENTS, RELOC, READONLY, DEBUGGING 136 29 .debug_info 0440b0df 00000000 00000000 006d3568 2**0 137 CONTENTS, RELOC, READONLY, DEBUGGING 138 30 .debug_abbrev 0022a83b 00000000 00000000 04ade647 2**0 139 CONTENTS, READONLY, DEBUGGING 140 31 .debug_line 004ead0d 00000000 00000000 04d08e82 2**0 141 CONTENTS, RELOC, READONLY, DEBUGGING 142 32 .debug_frame 0010a960 00000000 00000000 051f3b90 2**2 143 CONTENTS, RELOC, READONLY, DEBUGGING 144 33 .debug_str 001b442d 00000000 00000000 052fe4f0 2**0 145 CONTENTS, READONLY, DEBUGGING 146 34 .debug_loc 007c7fa9 00000000 00000000 054b291d 2**0 147 CONTENTS, RELOC, READONLY, DEBUGGING 148 35 .debug_ranges 00098828 00000000 00000000 05c7a8c8 2**3 149 CONTENTS, RELOC, READONLY, DEBUGGING 150 151There is also the setup binary mentioned earlier. This is at 152arch/x86/boot/setup.bin and is about 12KB in size. It includes the command 153line and various settings need by the kernel. Arguably the boot loader should 154provide all of this also, but setting it up is some complex that the kernel 155helps by providing a head start. 156 157As you can see the code loads to address 0x01000000 and everything else 158follows after that. We could load this image using the 'bootelf' command but 159we would still need to provide the setup binary. This is not supported by 160U-Boot although I suppose you could mostly script it. This would permit the 161use of a relocatable kernel. 162 163All we need to boot is the vmlinux file and the setup.bin file. 164 165 166Create a FIT 167------------ 168 169To create a FIT you will need a source file describing what should go in the 170FIT. See kernel.its for an example for x86 and also instructions on setting 171the 'arch' value for booting 64-bit kernels if desired. Put this into a file 172called image.its. 173 174Note that setup is loaded to the special address of 0x90000 (a special address 175you just have to know) and the kernel is loaded to 0x01000000 (the address you 176saw above). This means that you will need to load your FIT to a different 177address so that U-Boot doesn't overwrite it when decompressing. Something like 1780x02000000 will do so you can set CONFIG_SYS_LOAD_ADDR to that. 179 180In that example the kernel is compressed with lzo. Also we need to provide a 181flat binary, not an ELF. So the steps needed to set things are are: 182 183 # Create a flat binary 184 objcopy -O binary vmlinux vmlinux.bin 185 186 # Compress it into LZO format 187 lzop vmlinux.bin 188 189 # Build a FIT image 190 mkimage -f image.its image.fit 191 192(be careful to run the mkimage from your U-Boot tools directory since it 193will have x86_setup support.) 194 195You can take a look at the resulting fit file if you like: 196 197$ dumpimage -l image.fit 198FIT description: Simple image with single Linux kernel on x86 199Created: Tue Oct 7 10:57:24 2014 200 Image 0 (kernel) 201 Description: Vanilla Linux kernel 202 Created: Tue Oct 7 10:57:24 2014 203 Type: Kernel Image 204 Compression: lzo compressed 205 Data Size: 4591767 Bytes = 4484.15 kB = 4.38 MB 206 Architecture: Intel x86 207 OS: Linux 208 Load Address: 0x01000000 209 Entry Point: 0x00000000 210 Hash algo: sha1 211 Hash value: 446b5163ebfe0fb6ee20cbb7a8501b263cd92392 212 Image 1 (setup) 213 Description: Linux setup.bin 214 Created: Tue Oct 7 10:57:24 2014 215 Type: x86 setup.bin 216 Compression: uncompressed 217 Data Size: 12912 Bytes = 12.61 kB = 0.01 MB 218 Hash algo: sha1 219 Hash value: a1f2099cf47ff9816236cd534c77af86e713faad 220 Default Configuration: 'config-1' 221 Configuration 0 (config-1) 222 Description: Boot Linux kernel 223 Kernel: kernel 224 225 226Booting the FIT 227--------------- 228 229To make it boot you need to load it and then use 'bootm' to boot it. A 230suitable script to do this from a network server is: 231 232 bootp 233 tftp image.fit 234 bootm 235 236This will load the image from the network and boot it. The command line (from 237the 'bootargs' environment variable) will be passed to the kernel. 238 239If you want a ramdisk you can add it as normal with FIT. If you want a device 240tree then x86 doesn't normally use those - it has ACPI instead. 241 242 243Why Bother? 244----------- 245 2461. It demystifies the process of booting an x86 kernel 2472. It allows use of the standard U-Boot boot file format 2483. It allows U-Boot to perform decompression - problems will provide an error 249message and you are still in the boot loader. It is possible to investigate. 2504. It avoids all the pre-loader code in the kernel which is quite complex to 251follow 2525. You can use verified/secure boot and other features which haven't yet been 253added to the pre-Linux 2546. It makes x86 more like other architectures in the way it boots a kernel. 255You can potentially use the same file format for the kernel, and the same 256procedure for building and packaging it. 257 258 259References 260---------- 261 262In the Linux kernel, Documentation/x86/boot.txt defines the boot protocol for 263the kernel including the setup.bin format. This is handled in U-Boot in 264arch/x86/lib/zimage.c and arch/x86/lib/bootm.c. 265 266Various files in the same directory as this file describe the FIT format. 267 268 269-- 270Simon Glass 271sjg@chromium.org 2727-Oct-2014 273