| Booting Linux on x86 with FIT |
| ============================= |
| |
| Background |
| ---------- |
| |
| (corrections to the text below are welcome) |
| |
| Generally Linux x86 uses its own very complex booting method. There is a setup |
| binary which contains all sorts of parameters and a compressed self-extracting |
| binary for the kernel itself, often with a small built-in serial driver to |
| display decompression progress. |
| |
| The x86 CPU has various processor modes. I am no expert on these, but my |
| understanding is that an x86 CPU (even a really new one) starts up in a 16-bit |
| 'real' mode where only 1MB of memory is visible, moves to 32-bit 'protected' |
| mode where 4GB is visible (or more with special memory access techniques) and |
| then to 64-bit 'long' mode if 64-bit execution is required. |
| |
| Partly the self-extracting nature of Linux was introduced to cope with boot |
| loaders that were barely capable of loading anything. Even changing to 32-bit |
| mode was something of a challenge, so putting this logic in the kernel seemed |
| to make sense. |
| |
| Bit by bit more and more logic has been added to this post-boot pre-Linux |
| wrapper: |
| |
| - Changing to 32-bit mode |
| - Decompression |
| - Serial output (with drivers for various chips) |
| - Load address randomisation |
| - Elf loader complete with relocation (for the above) |
| - Random number generator via 3 methods (again for the above) |
| - Some sort of EFI mini-loader (1000+ glorious lines of code) |
| - Locating and tacking on a device tree and ramdisk |
| |
| To my mind, if you sit back and look at things from first principles, this |
| doesn't make a huge amount of sense. Any boot loader worth its salts already |
| has most of the above features and more besides. The boot loader already knows |
| the layout of memory, has a serial driver, can decompress things, includes an |
| ELF loader and supports device tree and ramdisks. The decision to duplicate |
| all these features in a Linux wrapper caters for the lowest common |
| denominator: a boot loader which consists of a BIOS call to load something off |
| disk, followed by a jmp instruction. |
| |
| (Aside: On ARM systems, we worry that the boot loader won't know where to load |
| the kernel. It might be easier to just provide that information in the image, |
| or in the boot loader rather than adding a self-relocator to put it in the |
| right place. Or just use ELF? |
| |
| As a result, the x86 kernel boot process is needlessly complex. The file |
| format is also complex, and obfuscates the contents to a degree that it is |
| quite a challenge to extract anything from it. This bzImage format has become |
| so prevalent that is actually isn't possible to produce the 'raw' kernel build |
| outputs with the standard Makefile (as it is on ARM for example, at least at |
| the time of writing). |
| |
| This document describes an alternative boot process which uses simple raw |
| images which are loaded into the right place by the boot loader and then |
| executed. |
| |
| |
| Build the kernel |
| ---------------- |
| |
| Note: these instructions assume a 32-bit kernel. U-Boot also supports directly |
| booting a 64-bit kernel by jumping into 64-bit mode first (see below). |
| |
| You can build the kernel as normal with 'make'. This will create a file called |
| 'vmlinux'. This is a standard ELF file and you can look at it if you like: |
| |
| $ objdump -h vmlinux |
| |
| vmlinux: file format elf32-i386 |
| |
| Sections: |
| Idx Name Size VMA LMA File off Algn |
| 0 .text 00416850 81000000 01000000 00001000 2**5 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE |
| 1 .notes 00000024 81416850 01416850 00417850 2**2 |
| CONTENTS, ALLOC, LOAD, READONLY, CODE |
| 2 __ex_table 00000c50 81416880 01416880 00417880 2**3 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 3 .rodata 00154b9e 81418000 01418000 00419000 2**5 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 4 __bug_table 0000597c 8156cba0 0156cba0 0056dba0 2**0 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 5 .pci_fixup 00001b80 8157251c 0157251c 0057351c 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 6 .tracedata 00000024 8157409c 0157409c 0057509c 2**0 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 7 __ksymtab 00007ec0 815740c0 015740c0 005750c0 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 8 __ksymtab_gpl 00004a28 8157bf80 0157bf80 0057cf80 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 9 __ksymtab_strings 0001d6fc 815809a8 015809a8 005819a8 2**0 |
| CONTENTS, ALLOC, LOAD, READONLY, DATA |
| 10 __init_rodata 00001c3c 8159e0a4 0159e0a4 0059f0a4 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 11 __param 00000ff0 8159fce0 0159fce0 005a0ce0 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 12 __modver 00000330 815a0cd0 015a0cd0 005a1cd0 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 13 .data 00063000 815a1000 015a1000 005a2000 2**12 |
| CONTENTS, ALLOC, LOAD, RELOC, DATA |
| 14 .init.text 0002f104 81604000 01604000 00605000 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE |
| 15 .init.data 00040cdc 81634000 01634000 00635000 2**12 |
| CONTENTS, ALLOC, LOAD, RELOC, DATA |
| 16 .x86_cpu_dev.init 0000001c 81674cdc 01674cdc 00675cdc 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 17 .altinstructions 0000267c 81674cf8 01674cf8 00675cf8 2**0 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 18 .altinstr_replacement 00000942 81677374 01677374 00678374 2**0 |
| CONTENTS, ALLOC, LOAD, READONLY, CODE |
| 19 .iommu_table 00000014 81677cb8 01677cb8 00678cb8 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 20 .apicdrivers 00000004 81677cd0 01677cd0 00678cd0 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, DATA |
| 21 .exit.text 00001a80 81677cd8 01677cd8 00678cd8 2**0 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE |
| 22 .data..percpu 00007880 8167a000 0167a000 0067b000 2**12 |
| CONTENTS, ALLOC, LOAD, RELOC, DATA |
| 23 .smp_locks 00003000 81682000 01682000 00683000 2**2 |
| CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA |
| 24 .bss 000a1000 81685000 01685000 00686000 2**12 |
| ALLOC |
| 25 .brk 00424000 81726000 01726000 00686000 2**0 |
| ALLOC |
| 26 .comment 00000049 00000000 00000000 00686000 2**0 |
| CONTENTS, READONLY |
| 27 .GCC.command.line 0003e055 00000000 00000000 00686049 2**0 |
| CONTENTS, READONLY |
| 28 .debug_aranges 0000f4c8 00000000 00000000 006c40a0 2**3 |
| CONTENTS, RELOC, READONLY, DEBUGGING |
| 29 .debug_info 0440b0df 00000000 00000000 006d3568 2**0 |
| CONTENTS, RELOC, READONLY, DEBUGGING |
| 30 .debug_abbrev 0022a83b 00000000 00000000 04ade647 2**0 |
| CONTENTS, READONLY, DEBUGGING |
| 31 .debug_line 004ead0d 00000000 00000000 04d08e82 2**0 |
| CONTENTS, RELOC, READONLY, DEBUGGING |
| 32 .debug_frame 0010a960 00000000 00000000 051f3b90 2**2 |
| CONTENTS, RELOC, READONLY, DEBUGGING |
| 33 .debug_str 001b442d 00000000 00000000 052fe4f0 2**0 |
| CONTENTS, READONLY, DEBUGGING |
| 34 .debug_loc 007c7fa9 00000000 00000000 054b291d 2**0 |
| CONTENTS, RELOC, READONLY, DEBUGGING |
| 35 .debug_ranges 00098828 00000000 00000000 05c7a8c8 2**3 |
| CONTENTS, RELOC, READONLY, DEBUGGING |
| |
| There is also the setup binary mentioned earlier. This is at |
| arch/x86/boot/setup.bin and is about 12KB in size. It includes the command |
| line and various settings need by the kernel. Arguably the boot loader should |
| provide all of this also, but setting it up is some complex that the kernel |
| helps by providing a head start. |
| |
| As you can see the code loads to address 0x01000000 and everything else |
| follows after that. We could load this image using the 'bootelf' command but |
| we would still need to provide the setup binary. This is not supported by |
| U-Boot although I suppose you could mostly script it. This would permit the |
| use of a relocatable kernel. |
| |
| All we need to boot is the vmlinux file and the setup.bin file. |
| |
| |
| Create a FIT |
| ------------ |
| |
| To create a FIT you will need a source file describing what should go in the |
| FIT. See kernel.its for an example for x86 and also instructions on setting |
| the 'arch' value for booting 64-bit kernels if desired. Put this into a file |
| called image.its. |
| |
| Note that setup is loaded to the special address of 0x90000 (a special address |
| you just have to know) and the kernel is loaded to 0x01000000 (the address you |
| saw above). This means that you will need to load your FIT to a different |
| address so that U-Boot doesn't overwrite it when decompressing. Something like |
| 0x02000000 will do so you can set CONFIG_SYS_LOAD_ADDR to that. |
| |
| In that example the kernel is compressed with lzo. Also we need to provide a |
| flat binary, not an ELF. So the steps needed to set things are are: |
| |
| # Create a flat binary |
| objcopy -O binary vmlinux vmlinux.bin |
| |
| # Compress it into LZO format |
| lzop vmlinux.bin |
| |
| # Build a FIT image |
| mkimage -f image.its image.fit |
| |
| (be careful to run the mkimage from your U-Boot tools directory since it |
| will have x86_setup support.) |
| |
| You can take a look at the resulting fit file if you like: |
| |
| $ dumpimage -l image.fit |
| FIT description: Simple image with single Linux kernel on x86 |
| Created: Tue Oct 7 10:57:24 2014 |
| Image 0 (kernel@1) |
| Description: Vanilla Linux kernel |
| Created: Tue Oct 7 10:57:24 2014 |
| Type: Kernel Image |
| Compression: lzo compressed |
| Data Size: 4591767 Bytes = 4484.15 kB = 4.38 MB |
| Architecture: Intel x86 |
| OS: Linux |
| Load Address: 0x01000000 |
| Entry Point: 0x00000000 |
| Hash algo: sha1 |
| Hash value: 446b5163ebfe0fb6ee20cbb7a8501b263cd92392 |
| Image 1 (setup@1) |
| Description: Linux setup.bin |
| Created: Tue Oct 7 10:57:24 2014 |
| Type: x86 setup.bin |
| Compression: uncompressed |
| Data Size: 12912 Bytes = 12.61 kB = 0.01 MB |
| Hash algo: sha1 |
| Hash value: a1f2099cf47ff9816236cd534c77af86e713faad |
| Default Configuration: 'config@1' |
| Configuration 0 (config@1) |
| Description: Boot Linux kernel |
| Kernel: kernel@1 |
| |
| |
| Booting the FIT |
| --------------- |
| |
| To make it boot you need to load it and then use 'bootm' to boot it. A |
| suitable script to do this from a network server is: |
| |
| bootp |
| tftp image.fit |
| bootm |
| |
| This will load the image from the network and boot it. The command line (from |
| the 'bootargs' environment variable) will be passed to the kernel. |
| |
| If you want a ramdisk you can add it as normal with FIT. If you want a device |
| tree then x86 doesn't normally use those - it has ACPI instead. |
| |
| |
| Why Bother? |
| ----------- |
| |
| 1. It demystifies the process of booting an x86 kernel |
| 2. It allows use of the standard U-Boot boot file format |
| 3. It allows U-Boot to perform decompression - problems will provide an error |
| message and you are still in the boot loader. It is possible to investigate. |
| 4. It avoids all the pre-loader code in the kernel which is quite complex to |
| follow |
| 5. You can use verified/secure boot and other features which haven't yet been |
| added to the pre-Linux |
| 6. It makes x86 more like other architectures in the way it boots a kernel. |
| You can potentially use the same file format for the kernel, and the same |
| procedure for building and packaging it. |
| |
| |
| References |
| ---------- |
| |
| In the Linux kernel, Documentation/x86/boot.txt defines the boot protocol for |
| the kernel including the setup.bin format. This is handled in U-Boot in |
| arch/x86/lib/zimage.c and arch/x86/lib/bootm.c. |
| |
| Various files in the same directory as this file describe the FIT format. |
| |
| |
| -- |
| Simon Glass |
| sjg@chromium.org |
| 7-Oct-2014 |