coreboot-kgpe-d16/Documentation/lessons/lesson1.md

coreboot lesson 1 - Starting from scratch

From a fresh Ubuntu 16.04 or 18.04 install, here are all the steps required for a very basic build:

Download, configure, and build coreboot

Step 1 - Install tools and libraries needed for coreboot

$ sudo apt-get install -y bison build-essential curl flex git gnat libncurses5-dev m4 zlib1g-dev

Step 2 - Download coreboot source tree

$ git clone https://review.coreboot.org/coreboot
$ cd coreboot

Step 3 - Build the coreboot toolchain

Please note that this can take a significant amount of time

$ make crossgcc-i386 CPUS=$(nproc)

Also note that you can possibly use your system toolchain, but the results are not reproducible, and may have issues, so this is not recommended. See step 5 to use your system toolchain.

Step 4 - Build the payload - coreinfo

$ make -C payloads/coreinfo olddefconfig
$ make -C payloads/coreinfo

Step 5 - Configure the build

Configure your mainboard

$ make menuconfig
   select 'Mainboard' menu
   Beside 'Mainboard vendor' should be '(Emulation)'
   Beside 'Mainboard model' should be 'QEMU x86 i440fx/piix4'
   select < Exit >

These should be the default selections, so if anything else was set, run make distclean to remove your old config file and start over.

Optionally use your system toolchain (Again, not recommended)

   select 'General Setup' menu
   select 'Allow building with any toolchain'
   select < Exit >

Select the payload

   select 'Payload' menu
   select 'Add a Payload'
   choose 'An Elf executable payload'
   select 'Payload path and filename'
   enter 'payloads/coreinfo/build/coreinfo.elf'
   select < Exit >
   select < Exit >
   select < Yes >

check your configuration (optional step):

$ make savedefconfig
$ cat defconfig

There should only be two lines (or 3 if you're using the system toolchain):

CONFIG_PAYLOAD_ELF=y
CONFIG_PAYLOAD_FILE="payloads/coreinfo/build/coreinfo.elf"

Step 6 - build coreboot

$ make

At the end of the build, you should see:

Build emulation/qemu-i440fx (QEMU x86 i440fx/piix4)

This means your build was successful. The output from the build is in the build directory. build/coreboot.rom is the full rom file.

Test the image using QEMU

Step 7 - Install QEMU

$ sudo apt-get install -y qemu

Step 8 - Run QEMU

Start QEMU, and point it to the ROM you just built:

$ qemu-system-x86_64 -bios build/coreboot.rom -serial stdio

You should see the serial output of coreboot in the original console window, and a new window will appear running the coreinfo payload.

Summary

Step 1 summary - Install tools and libraries needed for coreboot

You installed the minimum additional requirements for ubuntu to download and build coreboot. Ubuntu already has most of the other tools that would be required installed by default.

• build-essential is the basic tools for doing builds. It comes pre-installed on some Ubuntu flavors, and not on others.
• git is needed to download coreboot from the coreboot git repository.
• libncurses5-dev is needed to build the menu for 'make menuconfig'
• m4, bison, curl, flex, zlib1g-dev, gcc, gnat and g++ or clang are needed to build the coreboot toolchain. gcc and gnat have to be of the same version.

If you started with a different distribution, you might need to install many other items which vary by distribution.

Step 2 summary - Download coreboot source tree

This will download a 'read-only' copy of the coreboot tree. This just means that if you made changes to the coreboot tree, you couldn't immediately contribute them back to the community. To pull a copy of coreboot that would allow you to contribute back, you would first need to sign up for an account on gerrit.

Step 3 summary - Build the coreboot toolchain.

This builds one of the coreboot cross-compiler toolchains for X86 platforms. Because of the variability of compilers and the other required tools between the various operating systems that coreboot can be built on, coreboot supplies and uses its own cross-compiler toolchain to build the binaries that end up as part of the coreboot ROM. The toolchain provided by the operating system (the 'host toolchain') is used to build various tools that will run on the local system during the build process.

Step 4 summary - Build the payload

To actually do anything useful with coreboot, you need to build a payload to include in the rom. The idea behind coreboot is that it does the minimum amount possible before passing control of the machine to a payload. There are various payloads such as grub or SeaBIOS that are typically used to boot the operating system. Instead, we used coreinfo, a small demonstration payload that allows the user to look at various things such as memory and the contents of coreboot's cbfs - the pieces that make up the coreboot rom.

Step 5 summary - Configure the build

This step configures coreboot's build options using the menuconfig interface to Kconfig. Kconfig is the same configuration program used by the linux kernel. It allows you to enable, disable, and change various values to control the coreboot build process, including which mainboard(motherboard) to use, which toolchain to use, and how the runtime debug console should be presented and saved. Anytime you change mainboards in Kconfig, you should always run make distclean before running make menuconfig. Due to the way that Kconfig works, values will be kept from the previous mainboard if you skip the clean step. This leads to a hybrid configuration which may or may not work as expected.

Step 6 summary - Build coreboot

You may notice that a number of other pieces are downloaded at the beginning of the build process. These are the git submodules used in various coreboot builds. By default, the blobs submodule is not downloaded. This git submodule may be required for other builds for microcode or other binaries. To enable downloading this submodule, select the option "Allow use of binary-only repository" in the "General Setup" menu of Kconfig This attempts to build the coreboot rom. The rom file itself ends up in the build directory as 'coreboot.rom'. At the end of the build process, the build displayed the contents of the rom file.

Step 7 summary - Install QEMU

QEMU is a processor emulator which we can use to show coreboot

Step 8 summary - Run QEMU

Here's the command line broken down:

• qemu-system-x86_64 This starts the QEMU emulator with the i440FX host PCI bridge and PIIX3 PCI to ISA bridge.
• -bios build/coreboot.rom Use the bios rom image that we just built. If this is left off, the standard SeaBIOS image that comes with QEMU is used.
• -serial stdio Send the serial output to the console. This allows you to view the coreboot debug output.