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diff --git a/doc/uImage.FIT/x86-fit-boot.txt b/doc/uImage.FIT/x86-fit-boot.txt new file mode 100644 index 0000000..61c10ff --- /dev/null +++ b/doc/uImage.FIT/x86-fit-boot.txt @@ -0,0 +1,276 @@ +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 does not currently +support booting a 64-bit kernel as it has no way of going into 64-bit mode on +x86. + +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. 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. + +The procedure for entering 64-bit mode on x86 seems to be described here: + + http://wiki.osdev.org/64-bit_Higher_Half_Kernel_with_GRUB_2 + +Various files in the same directory as this file describe the FIT format. + + +-- +Simon Glass +sjg@chromium.org +7-Oct-2014 |