Documentation/x86: explain LINUX_EFI_INITRD_MEDIA_GUID

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.

[Bagas: Don't use :ref: link to EFI stub documentation and refer to
OVMF/edk2 implementation]

Signed-off-by: Hugo Osvaldo Barrera <hugo@whynothugo.nl>
Link: https://lore.kernel.org/r/20250428131206.8656-2-hugo@whynothugo.nl
Co-developed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251013085718.27085-1-bagasdotme@gmail.com>

2 files changed, 34 insertions, 9 deletions

diff --git a/Documentation/admin-guide/efi-stub.rst b/Documentation/admin-guide/efi-stub.rst
index 090f3a185e18..f8e7407698bd 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 77e6163288db..32eea3d2807e 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.
-
-[0] https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0
+   The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF
+   entry point described below.
+
+.. _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