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

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Message-ID: <0b61b6f3-f11a-494c-9d4e-94ca19fd1eb6@infradead.org>
Date: Tue, 9 Sep 2025 20:25:34 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Bagas Sanjaya <bagasdotme@...il.com>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 Linux Documentation <linux-doc@...r.kernel.org>,
 Linux EFI <linux-efi@...r.kernel.org>
Cc: Ard Biesheuvel <ardb@...nel.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] x86/Documentation: explain LINUX_EFI_INITRD_MEDIA_GUID

Hi Bagas,

This mostly looks good to me. I have a couple of small comments
below.


On 9/9/25 6:57 PM, Bagas Sanjaya 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]
> Co-developed-by: Bagas Sanjaya <bagasdotme@...il.com>
> Signed-off-by: Bagas Sanjaya <bagasdotme@...il.com>
> ---
>  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

> +.. _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.

drivers/firmware/efi/libstub/efi-stub-helper.c (line 509) says LoadFile2
protocol. Is that the same as the LoadFile function?

https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0
(the link below) also says LoadFile2() 4 times (and LoadFile 0 times).

thanks.
-- 
~Randy