Arch4021: Introductory UEFI 04 04 UEFI BDS

Summary

The video script explores the UEFI boot device selection phase, focusing on the Boot Device Selection (BDS) process and the role of UEFI interfaces in loading operating systems. It highlights how BDS utilizes UEFI services to detect bootable devices and load OS or transient applications, with specific mention of the .efi OS loader format. The script also discusses the boot order, defined by UEFI variables, and the confusion between bootloaders and operating systems, particularly regarding the ExitBootServices() function, which marks the transition from boot services to operating system control.

Takeaways

• 😀 UEFI (Unified Extensible Firmware Interface) is used in the BDS (Boot Device Selection) phase to provide boot services and runtime services.
• 😀 The UEFI specification is large, but its interfaces are used relatively late in the boot process after hardware components are correctly configured by vendors.
• 😀 BDS is responsible for loading the operating system or running transient applications like memtest or the UEFI shell.
• 😀 The BDS phase can detect all devices and allows for selecting the boot media and determining which operating system to boot.
• 😀 The operating system loader is typically in a portable executable format with a .efi extension (e.g., bootx64.efi for 64-bit systems).
• 😀 Boot order and BDS behavior are controlled by UEFI variables, which can be modified through the operating system, UEFI setup, or a GUI in modern systems.
• 😀 By default, BDS looks for an EFI boot file (e.g., efi/boot/bootx64.efi) on the first bootable partition and executes the first file it finds.
• 😀 The BDS phase typically ends with the execution of ExitBootServices(), which makes boot services unavailable after the OS starts.
• 😀 A key challenge is the confusion between OS loader and kernel integration, as many operating systems merge these components.
• 😀 There can be technical issues when using external bootloaders (e.g., GRUB, SeaBIOS, iPXE), as the responsibility for calling ExitBootServices() may be unclear—whether it should be handled by the bootloader or the operating system.

Q & A

What is the role of the Boot Device Selection (BDS) phase in UEFI?

The BDS phase is responsible for selecting and loading the operating system or other applications, such as memtest or the UEFI shell, by interacting with the UEFI interfaces and services. It allows for boot device selection and the management of the boot process.

How does UEFI specification impact the BDS phase?

UEFI defines the boot services and runtime services that the BDS phase uses. Although UEFI is a comprehensive specification, its use is relatively late in the boot process, with the underlying components developed closely with hardware vendors to expose UEFI interfaces correctly.

What is the typical file format for an operating system loader in UEFI?

The operating system loader in UEFI is typically a portable executable (PE) file with the .efi extension. For example, for x86-64 architecture, the file is usually named bootx64.efi.

How does BDS determine which boot media to select?

BDS iterates through all bootable partitions, looking for the bootloader file (such as efi/boot/bootx64.efi) on the first partition. Once it finds a valid bootloader, it executes it to load the operating system.

What is the role of UEFI variables in the BDS phase?

UEFI variables define the boot order and behavior during the BDS phase. These variables can be modified either by the operating system, through the UEFI setup menu (which might be a graphical user interface in modern systems), or directly by the user.

What happens after BDS selects and loads the OS loader?

After selecting and loading the OS loader, the BDS phase typically ends with the `ExitBootServices()` function. This marks the end of the boot services phase and transitions the system to the operating system runtime environment.

Why is the `ExitBootServices()` function important?

`ExitBootServices()` is crucial because it ends the availability of boot services. Once executed, the system moves from the pre-boot environment (BDS) to the operating system runtime, where boot services are no longer accessible.

What challenge is associated with integrating bootloaders like GRUB or SeaBIOS with the OS kernel?

The challenge lies in the fact that many operating systems combine the OS loader with the kernel. This makes it unclear whether the bootloader (e.g., GRUB, SeaBIOS, iPXE) should call `ExitBootServices()` or if the operating system itself should handle this step after the loader is executed.

How does the OS loader interact with the kernel during the boot process?

The OS loader is responsible for loading the kernel into memory. After this, the kernel typically takes over and begins executing the system, often invoking `ExitBootServices()` to complete the transition to the OS runtime.

What happens if the OS loader and kernel are integrated into a single file?

When the OS loader and kernel are integrated into one file, the bootloader might not need to explicitly call `ExitBootServices()`. Instead, the kernel itself handles this step after it is loaded, eliminating potential confusion about the bootloader's responsibilities.