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Simon Glass90268b82014-10-19 21:11:24 -06001Booting 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 does not currently
67support booting a 64-bit kernel as it has no way of going into 64-bit mode on
68x86.
69
70You can build the kernel as normal with 'make'. This will create a file called
71'vmlinux'. This is a standard ELF file and you can look at it if you like:
72
73$ objdump -h vmlinux
74
75vmlinux: file format elf32-i386
76
77Sections:
78Idx Name Size VMA LMA File off Algn
79 0 .text 00416850 81000000 01000000 00001000 2**5
80 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
81 1 .notes 00000024 81416850 01416850 00417850 2**2
82 CONTENTS, ALLOC, LOAD, READONLY, CODE
83 2 __ex_table 00000c50 81416880 01416880 00417880 2**3
84 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
85 3 .rodata 00154b9e 81418000 01418000 00419000 2**5
86 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
87 4 __bug_table 0000597c 8156cba0 0156cba0 0056dba0 2**0
88 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
89 5 .pci_fixup 00001b80 8157251c 0157251c 0057351c 2**2
90 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
91 6 .tracedata 00000024 8157409c 0157409c 0057509c 2**0
92 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
93 7 __ksymtab 00007ec0 815740c0 015740c0 005750c0 2**2
94 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
95 8 __ksymtab_gpl 00004a28 8157bf80 0157bf80 0057cf80 2**2
96 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
97 9 __ksymtab_strings 0001d6fc 815809a8 015809a8 005819a8 2**0
98 CONTENTS, ALLOC, LOAD, READONLY, DATA
99 10 __init_rodata 00001c3c 8159e0a4 0159e0a4 0059f0a4 2**2
100 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
101 11 __param 00000ff0 8159fce0 0159fce0 005a0ce0 2**2
102 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
103 12 __modver 00000330 815a0cd0 015a0cd0 005a1cd0 2**2
104 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
105 13 .data 00063000 815a1000 015a1000 005a2000 2**12
106 CONTENTS, ALLOC, LOAD, RELOC, DATA
107 14 .init.text 0002f104 81604000 01604000 00605000 2**2
108 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
109 15 .init.data 00040cdc 81634000 01634000 00635000 2**12
110 CONTENTS, ALLOC, LOAD, RELOC, DATA
111 16 .x86_cpu_dev.init 0000001c 81674cdc 01674cdc 00675cdc 2**2
112 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
113 17 .altinstructions 0000267c 81674cf8 01674cf8 00675cf8 2**0
114 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
115 18 .altinstr_replacement 00000942 81677374 01677374 00678374 2**0
116 CONTENTS, ALLOC, LOAD, READONLY, CODE
117 19 .iommu_table 00000014 81677cb8 01677cb8 00678cb8 2**2
118 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
119 20 .apicdrivers 00000004 81677cd0 01677cd0 00678cd0 2**2
120 CONTENTS, ALLOC, LOAD, RELOC, DATA
121 21 .exit.text 00001a80 81677cd8 01677cd8 00678cd8 2**0
122 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
123 22 .data..percpu 00007880 8167a000 0167a000 0067b000 2**12
124 CONTENTS, ALLOC, LOAD, RELOC, DATA
125 23 .smp_locks 00003000 81682000 01682000 00683000 2**2
126 CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
127 24 .bss 000a1000 81685000 01685000 00686000 2**12
128 ALLOC
129 25 .brk 00424000 81726000 01726000 00686000 2**0
130 ALLOC
131 26 .comment 00000049 00000000 00000000 00686000 2**0
132 CONTENTS, READONLY
133 27 .GCC.command.line 0003e055 00000000 00000000 00686049 2**0
134 CONTENTS, READONLY
135 28 .debug_aranges 0000f4c8 00000000 00000000 006c40a0 2**3
136 CONTENTS, RELOC, READONLY, DEBUGGING
137 29 .debug_info 0440b0df 00000000 00000000 006d3568 2**0
138 CONTENTS, RELOC, READONLY, DEBUGGING
139 30 .debug_abbrev 0022a83b 00000000 00000000 04ade647 2**0
140 CONTENTS, READONLY, DEBUGGING
141 31 .debug_line 004ead0d 00000000 00000000 04d08e82 2**0
142 CONTENTS, RELOC, READONLY, DEBUGGING
143 32 .debug_frame 0010a960 00000000 00000000 051f3b90 2**2
144 CONTENTS, RELOC, READONLY, DEBUGGING
145 33 .debug_str 001b442d 00000000 00000000 052fe4f0 2**0
146 CONTENTS, READONLY, DEBUGGING
147 34 .debug_loc 007c7fa9 00000000 00000000 054b291d 2**0
148 CONTENTS, RELOC, READONLY, DEBUGGING
149 35 .debug_ranges 00098828 00000000 00000000 05c7a8c8 2**3
150 CONTENTS, RELOC, READONLY, DEBUGGING
151
152There is also the setup binary mentioned earlier. This is at
153arch/x86/boot/setup.bin and is about 12KB in size. It includes the command
154line and various settings need by the kernel. Arguably the boot loader should
155provide all of this also, but setting it up is some complex that the kernel
156helps by providing a head start.
157
158As you can see the code loads to address 0x01000000 and everything else
159follows after that. We could load this image using the 'bootelf' command but
160we would still need to provide the setup binary. This is not supported by
161U-Boot although I suppose you could mostly script it. This would permit the
162use of a relocatable kernel.
163
164All we need to boot is the vmlinux file and the setup.bin file.
165
166
167Create a FIT
168------------
169
170To create a FIT you will need a source file describing what should go in the
171FIT. See kernel.its for an example for x86. Put this into a file called
172image.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@1)
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@1)
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@1
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
266The procedure for entering 64-bit mode on x86 seems to be described here:
267
268 http://wiki.osdev.org/64-bit_Higher_Half_Kernel_with_GRUB_2
269
270Various files in the same directory as this file describe the FIT format.
271
272
273--
274Simon Glass
275sjg@chromium.org
2767-Oct-2014