On Wed, 10 Sept 2025 at 03:58, Bagas Sanjaya <bagasdotme@xxxxxxxxx> wrote: > > 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. > Better say 'using a Linux-specific UEFI protocol at boot time'. The boot services are a specific set of APIs none of which have anything to do with initrd loading. > 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 ... 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. > +``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. > + ... 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 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 > What is candyboot, and why are we adding this plug for it into the Linux documentation?
