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

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Message-ID: <20201105152836.GB4472@sequoia>
Date:   Thu, 5 Nov 2020 09:28:36 -0600
From:   Tyler Hicks <tyhicks@...ux.microsoft.com>
To:     Will Deacon <will@...nel.org>
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 2020-11-05 09:58:54, Will Deacon wrote:
> 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?

x86's equivalent was implemented by commit 516cbf3730c4 ("x86, bootup:
add built-in kernel command line for x86 (v2)"). To summarize, you have
to enable CONFIG_CMDLINE_BOOL and then that lets you define built-in
command line parameters in CONFIG_CMDLINE. However, it is backwards in
that the command line provided by the bootloader is appended onto the
end of CONFIG_CMDLINE.

This doesn't seem as useful to me because, using the crashkernel=
example from above, the bootloader provided crashkernel= value may need
to be overridden by the built-in command line to provide a different
crashkernel= value for the particular kernel build being booted. Most
kernel command line parameter parsers are implemented in a way that
supports multiple instances of the parameter while only honoring the
last instance.

Tyler

> 
> Will
>