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

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Message-Id: <4f36eee9-7e25-4e2f-ab9d-82f9732d0bf1@app.fastmail.com>
Date: Thu, 11 Sep 2025 16:07:33 +0200
From: "Hugo Osvaldo Barrera" <hugo@...nothugo.nl>
To: "Ard Biesheuvel" <ardb@...nel.org>
Cc: "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>, "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>
Subject: Re: [PATCH] x86/Documentation: explain LINUX_EFI_INITRD_MEDIA_GUID

On Thu, 11 Sep 2025, at 14:49, Ard Biesheuvel wrote:
> On Thu, 11 Sept 2025 at 13:23, Hugo Osvaldo Barrera <hugo@...nothugo.nl> wrote:
>>
>>
>>
>> On Thu, 11 Sep 2025, at 08:46, Ard Biesheuvel wrote:
>> > On Wed, 10 Sept 2025 at 03:58, Bagas Sanjaya <bagasdotme@...il.com> wrote:
>> >>
>> >> From: Hugo Osvaldo Barrera <hugo@...nothugo.nl>
>> >>
> ...
>> >> +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?
>>
>> It's a UEFI stub loader which can load the Linux kernel and provide it with an
>> initramfs using the above described protocol.
>>
>> The original version of this patch was based on my notes researching _how_
>> to implement this stub loader. The implementation is quite minimal, so I think
>> it serves as a useful reference example.
>>
>
> I think one example reference is sufficient, and I think piggybacking
> a plug of your own project onto a documentation refactoring patch is
> slightly dodgy, to be completely honest.

I don't know what kind of reaction you're expecting. The documentation
improvements were a side-effect of developing candyboot. You're free to
exclude the mention if you feel it is inadequate, but I don't appreciate
the personal attack.

> Where is candyboot used, and what does it add to the existing u-boot
> reference, which is the most widely used EFI implementation after EDK2
> for non-x86 systems?

candyboot can be used to produce a single bootable UEFI binary bundling
the kernel, cmdline and initrd. This binary can be signed to boot using
SecureBoot on regular consumer hardware. It is typically useful is
situations where u-boot isn't supported, and useless in most situations
where u-boot is usable.

The use-case is somewhat tangential here; the reference is merely as a
minimal (single file) reference implementation of the feature being
described here.

> If anything, we should be referring to the OVMF implementation here.

Agreed (I didn't know of its existence), here's a link to it:

https://github.com/tianocore/edk2/blob/502f0dfda4f2c4d1cc091f68b6467b6ef12cab45/OvmfPkg/QemuKernelLoaderFsDxe/QemuKernelLoaderFsDxe.c#L908

-- 
Hugo