linux-kernel - Re: [PATCH v3] Documentation/x86: explain LINUX_EFI_INITRD_MEDIA

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Message-ID: <CAMj1kXH5Y+mgT7LuQZOkAf5Ptpf=Gn3U=6XgrXxoe=irQCBnHw@mail.gmail.com>
Date: Tue, 14 Oct 2025 09:36:58 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Bagas Sanjaya <bagasdotme@...il.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	Linux Documentation <linux-doc@...r.kernel.org>, Linux EFI <linux-efi@...r.kernel.org>, 
	Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Hugo Osvaldo Barrera <hugo@...nothugo.nl>
Subject: Re: [PATCH v3] Documentation/x86: explain LINUX_EFI_INITRD_MEDIA_GUID

On Mon, 13 Oct 2025 at 10:57, Bagas Sanjaya <bagasdotme@...il.com> wrote:
>
> From: Hugo Osvaldo Barrera <hugo@...nothugo.nl>
>
> 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@...nothugo.nl>
> Link: https://lore.kernel.org/r/20250428131206.8656-2-hugo@whynothugo.nl
> [Bagas: Don't use :ref: link to EFI stub documentation and refer to
> OVMF/edk2 implementation]
> Co-developed-by: Bagas Sanjaya <bagasdotme@...il.com>
> Signed-off-by: Bagas Sanjaya <bagasdotme@...il.com>
> ---
> No changes since v2 [1].
>
> EFI/x86 maintainers: Would you like to apply this patch on tip/efi tree
> or let Jon handle it through docs-next instead?
>

Please take this via the documentation tree - thanks.

> [1]: https://lore.kernel.org/linux-doc/20250916073244.590483-1-bagasdotme@gmail.com/
>
>  Documentation/admin-guide/efi-stub.rst |  3 ++
>  Documentation/arch/x86/boot.rst        | 38 ++++++++++++++++++++------
>  2 files changed, 33 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/admin-guide/efi-stub.rst b/Documentation/admin-guide/efi-stub.rst
> index 090f3a185e1897..f8e7407698bd2a 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 a Linux-specific UEFI
> +   protocol at boot time. 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..32eea3d2807e1c 100644
> --- a/Documentation/arch/x86/boot.rst
> +++ b/Documentation/arch/x86/boot.rst
> @@ -1431,12 +1431,34 @@ 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 carries
> +implementations of the ``EFI_LOAD_FILE2`` protocol and the device path
> +protocol exposing the ``LINUX_EFI_INITRD_MEDIA_GUID`` vendor media device path.
> +In this case, a kernel booting via the EFI stub will invoke
> +``LoadFile2::LoadFile()`` method on the registered protocol to instruct the
> +firmware to load the initrd into a memory location chosen by the kernel/EFI
> +stub.
> +
> +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 OVMF implementation`_.
> +
> +.. _the original u-boot implementation: https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0
> +.. _the OVMF implementation: https://github.com/tianocore/edk2/blob/1780373897f12c25075f8883e073144506441168/OvmfPkg/LinuxInitrdDynamicShellCommand/LinuxInitrdDynamicShellCommand.c
>
> base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
> --
> An old man doll... just what I always wanted! - Clara
>
>