x86: Support loading kernel setup from a FIT

Add a new setup@ section to the FIT which can be used to provide a setup
binary for booting Linux on x86. This makes it possible to boot x86 from
a FIT.

Signed-off-by: Simon Glass <sjg@chromium.org>

1 files changed, 276 insertions, 0 deletions

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
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+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