From: Hugo Osvaldo Barrera <hugo@xxxxxxxxxxxxx> Since the Handover Protocol was deprecated, the recommended approach is to provide an initrd using a UEFI boot service with the LINUX_EFI_INITRD_MEDIA_GUID device path. Documentation for the new approach has been no more than an admonition with a link to an existing implementation. Provide a short explanation of this functionality, to ease future implementations without having to reverse engineer existing ones. Signed-off-by: Hugo Osvaldo Barrera <hugo@xxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20250428131206.8656-2-hugo@xxxxxxxxxxxxx [Bagas: Don't use :ref: link to EFI stub documentation] Co-developed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> Signed-off-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> --- Documentation/admin-guide/efi-stub.rst | 3 +++ Documentation/arch/x86/boot.rst | 35 ++++++++++++++++++++------ 2 files changed, 30 insertions(+), 8 deletions(-) diff --git a/Documentation/admin-guide/efi-stub.rst b/Documentation/admin-guide/efi-stub.rst index 090f3a185e1897..2f0f040f6913a4 100644 --- a/Documentation/admin-guide/efi-stub.rst +++ b/Documentation/admin-guide/efi-stub.rst @@ -79,6 +79,9 @@ because the image we're executing is interpreted by the EFI shell, which understands relative paths, whereas the rest of the command line is passed to bzImage.efi. +.. hint:: + It is also possible to provide an initrd using UEFI boot services. See + :ref:`pe-coff-entry-point` for details. The "dtb=" option ----------------- diff --git a/Documentation/arch/x86/boot.rst b/Documentation/arch/x86/boot.rst index 77e6163288db08..fadbe66517bdf2 100644 --- a/Documentation/arch/x86/boot.rst +++ b/Documentation/arch/x86/boot.rst @@ -1431,12 +1431,31 @@ The boot loader *must* fill out the following fields in bp:: All other fields should be zero. .. note:: - The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF - entry point, combined with the LINUX_EFI_INITRD_MEDIA_GUID based initrd - loading protocol (refer to [0] for an example of the bootloader side of - this), which removes the need for any knowledge on the part of the EFI - bootloader regarding the internal representation of boot_params or any - requirements/limitations regarding the placement of the command line - and ramdisk in memory, or the placement of the kernel image itself. + The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF + entry point described below. -[0] https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0 +.. _pe-coff-entry-point: + +PE/COFF entry point +=================== + +When compiled with ``CONFIG_EFI_STUB=y``, the kernel can be executed as a +regular PE/COFF binary. See Documentation/admin-guide/efi-stub.rst for +implementation details. + +The stub loader can request the initrd via a UEFI protocol. For this to work, +the firmware or bootloader needs to register a handle which implements the +``EFI_LOAD_FILE2`` protocol with the ``LINUX_EFI_INITRD_MEDIA_GUID`` device +path. In this case, a kernel booting via the EFI stub will use the ``LoadFile`` +function on the registered handle to obtain a reference to the initrd. + +This approach removes the need for any knowledge on the part of the EFI +bootloader regarding the internal representation of boot_params or any +requirements/limitations regarding the placement of the command line and +ramdisk in memory, or the placement of the kernel image itself. + +For sample implementations, refer to `the original u-boot implementation`_ or +`the implementation in candyboot`_. + +.. _the original u-boot implementation: https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0 +.. _the implementation in candyboot: https://git.sr.ht/~whynothugo/candyboot/tree/4097b2538d7f1cf85f03922bf42409490b666202/item/src/main.rs#L225 base-commit: f44a29784f685804d9970cfb0d3439c9e30981d7 -- An old man doll... just what I always wanted! - Clara
