linux-kernel - Re: [PATCH 0/2] arm64: Implement CONFIG_CMDLINE

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Date:   Thu, 5 Nov 2020 09:58:54 +0000
From:   Will Deacon <will@...nel.org>
To:     Tyler Hicks <tyhicks@...ux.microsoft.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Rob Herring <robh+dt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] arm64: Implement CONFIG_CMDLINE_EXTEND

On Wed, Nov 04, 2020 at 11:40:09PM -0600, Tyler Hicks wrote:
> On 2020-11-04 12:08:12, Will Deacon wrote:
> > On Tue, Nov 03, 2020 at 09:59:52AM -0600, Tyler Hicks wrote:
> > > On 2020-09-21 14:15:55, Tyler Hicks wrote:
> > > > Provide the CONFIG_CMDLINE_EXTEND config option for arm64 kernels. This
> > > > config option can be used to extend the kernel command line parameters,
> > > > specified by the bootloader, with additional command line parameters
> > > > specified in the kernel configuration.
> > > 
> > > Hi Catalin and Will - Friendly ping on this series now that we're
> > > on the other side of the 5.10 merge window. I hope it can be considered
> > > for 5.10+1. Let me know if I need to rebase/resubmit. Thanks!
> > 
> > Can you use bootconfig to achieve what you need?
> 
> Thanks for mentioning bootconfig. I hadn't considered it.
> 
> After reading the docs and code, I see a few reasons why I can't use it
> out of the box:
> 
>  1) It requires "bootconfig" to be appended to the kernel command line.
>     My proposed patch series makes it possible to append new options to
>     the kernel command line in situations where the bootloader is not
>     interactive. This presents a circular dependency problem for my use
>     case.
> 
>     A new config option could be added to force the enablement of
>     bootconfig but that would sort of be a single-use duplicate of
>     CONFIG_CMDLINE_EXTEND's functionality.
> 
>  2) Not all kernel command line options can be configured using
>     bootconfig. For example, the "nokaslr" and "crashkernel=" parameters
>     are parsed/handled before setup_boot_config() is called. KASLR can
>     be disabled via a kernel config change but there's no config option
>     equivalent for "crashkernel=". Changing the "crashkernel=" command
>     line option is something that I need to support because a
>     development/debug kernel build often requires a larger reservation
>     and we find ourselves adjusting the "crashkernel=" value fairly
>     often.
> 
>  3) External FIT image build systems do not yet support bootconfig since
>     it is so new. It is completely fair if you file this away in your
>     not-my-problem folder but simple kernel config modifications, as
>     needed for CONFIG_CMDLINE_EXTEND, are something that every image
>     build system is likely to support today.
> 
> All that said, I do really like the look of bootconfig. Unfortunately,
> it doesn't let me achieve everything I need.

Ok, well thanks for having a look. A follow-up question I have is how is
this handled on x86? They don't appear to have CONFIG_CMDLINE_EXTEND either
afaict. Is it because their bootloader story tends to be more uniform?

Will