LinuxBoot

The more things change, the more they stay the same. That may sound cliché, but it's still as true for the firmware that boots your operating system as it was in 2001 when Linux Journal first published Eric Biederman's "About LinuxBIOS ". LinuxBoot is the latest incarnation of an idea that has persisted for around two decades now: use Linux as your bootstrap. On most systems, firmware exists to put the hardware in a state where an operating system can take over. In some cases, the firmware and OS are closely intertwined and may even be the same binary; however, Linux-based systems generally have a firmware component that initializes hardware before loading the Linux kernel itself. This may include initialization of DRAM, storage and networking interfaces, as well as performing security-related functions prior to starting Linux. To provide some perspective, this pre-Linux setup could be done in 100 or so instructions in 1999; now it's more than a billion. Oftentimes it's suggested that Linux itself should be placed at the boot vector. That was the first attempt at LinuxBIOS on x86, and it looked something like this: This didn't get very far though. Linux was not at the point where it could fully bootstrap itself—for example, it lacked functionality to initialize DRAM. So LinuxBIOS, later renamed coreboot, implemented the minimum hardware initialization functionality needed to start a kernel. Today Linux is much more mature and can initialize much more—although not everything—on its own. A large part of this has to do with the need to integrate system and power management features, such as sleep states and CPU/device hotplug support, into the kernel for optimal performance and power-saving. Virtualization also has led to improvements in Linux's ability to boot itself. LinuxBoot replaces the late stages with a Linux kernel and initramfs, which are used to load and execute the next stage, whatever it may be and wherever it may come from. The Linux kernel included in LinuxBoot is called the "boot kernel" to distinguish it from the "target kernel" that is to be booted and may be something other than Linux. Bundling a Linux kernel with the firmware simplifies the firmware in two major ways. First, it eliminates the need for firmware to contain drivers for the ever-increasing variety of boot media, networking interfaces and peripherals. Second, we can use familiar tools from Linux userspace, such as wget and cryptsetup, to handle tasks like downloading a target OS kernel or accessing encrypted partitions, so that the late stages of firmware do not need to (re-)implement sophisticated tools and libraries. In addition to the boot path, there are runtime components bundled with firmware that handle errors and other hardware events. These are referred to as RAS (Reliability, Availability and Serviceability) features. RAS features often are implemented as high-priority, highly privileged interrupt handlers that run outside the context of the operating system—for example, in system management mode (SMM) on x86. This brings performance and security concerns that LinuxBoot is addressing by moving some RAS features into Linux. For more information, see Ron Minnich's ECC'17 talk "Let's Move SMM out of firmware and into the kernel ". The initramfs used by the boot kernel can be any Linux-compatible environment that the user can fit into the boot ROM. For ideas, see "Custom Embedded Linux Distributions" published by Linux Journal in February 2018. While LinuxBoot can benefit those who are familiar with Linux and want more control over their boot flow, companies with large deployments of Linux-based servers or products stand to gain the most. They usually have teams of engineers or entire organizations with expertise in developing, deploying and supporting Linux kernel and userland—their business depends on it after all. Replacing obscure and often closed firmware code with Linux enables organizations to leverage talent they already have