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100 lines
3.8 KiB
100 lines
3.8 KiB
================= |
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The EFI Boot Stub |
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================= |
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On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade |
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as a PE/COFF image, thereby convincing EFI firmware loaders to load |
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it as an EFI executable. The code that modifies the bzImage header, |
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along with the EFI-specific entry point that the firmware loader |
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jumps to are collectively known as the "EFI boot stub", and live in |
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arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c, |
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respectively. For ARM the EFI stub is implemented in |
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arch/arm/boot/compressed/efi-header.S and |
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arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared |
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between architectures is in drivers/firmware/efi/libstub. |
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For arm64, there is no compressed kernel support, so the Image itself |
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masquerades as a PE/COFF image and the EFI stub is linked into the |
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kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S |
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and drivers/firmware/efi/libstub/arm64-stub.c. |
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By using the EFI boot stub it's possible to boot a Linux kernel |
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without the use of a conventional EFI boot loader, such as grub or |
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elilo. Since the EFI boot stub performs the jobs of a boot loader, in |
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a certain sense it *IS* the boot loader. |
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The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option. |
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How to install bzImage.efi |
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-------------------------- |
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The bzImage located in arch/x86/boot/bzImage must be copied to the EFI |
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System Partition (ESP) and renamed with the extension ".efi". Without |
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the extension the EFI firmware loader will refuse to execute it. It's |
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not possible to execute bzImage.efi from the usual Linux file systems |
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because EFI firmware doesn't have support for them. For ARM the |
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arch/arm/boot/zImage should be copied to the system partition, and it |
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may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image |
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should be copied but not necessarily renamed. |
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Passing kernel parameters from the EFI shell |
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-------------------------------------------- |
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Arguments to the kernel can be passed after bzImage.efi, e.g.:: |
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fs0:> bzImage.efi console=ttyS0 root=/dev/sda4 |
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The "initrd=" option |
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-------------------- |
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Like most boot loaders, the EFI stub allows the user to specify |
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multiple initrd files using the "initrd=" option. This is the only EFI |
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stub-specific command line parameter, everything else is passed to the |
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kernel when it boots. |
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The path to the initrd file must be an absolute path from the |
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beginning of the ESP, relative path names do not work. Also, the path |
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is an EFI-style path and directory elements must be separated with |
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backslashes (\). For example, given the following directory layout:: |
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fs0:> |
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Kernels\ |
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bzImage.efi |
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initrd-large.img |
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Ramdisks\ |
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initrd-small.img |
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initrd-medium.img |
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to boot with the initrd-large.img file if the current working |
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directory is fs0:\Kernels, the following command must be used:: |
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fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img |
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Notice how bzImage.efi can be specified with a relative path. That's |
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because the image we're executing is interpreted by the EFI shell, |
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which understands relative paths, whereas the rest of the command line |
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is passed to bzImage.efi. |
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The "dtb=" option |
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----------------- |
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For the ARM and arm64 architectures, a device tree must be provided to |
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the kernel. Normally firmware shall supply the device tree via the |
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EFI CONFIGURATION TABLE. However, the "dtb=" command line option can |
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be used to override the firmware supplied device tree, or to supply |
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one when firmware is unable to. |
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Please note: Firmware adds runtime configuration information to the |
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device tree before booting the kernel. If dtb= is used to override |
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the device tree, then any runtime data provided by firmware will be |
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lost. The dtb= option should only be used either as a debug tool, or |
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as a last resort when a device tree is not provided in the EFI |
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CONFIGURATION TABLE. |
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"dtb=" is processed in the same manner as the "initrd=" option that is |
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described above.
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