Re: custom kernel config

[Efraim Flashner <efraim@flashner.co.il>]

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On Thu, Apr 04, 2019 at 01:44:48AM -0700, Chris Marusich wrote:
> Efraim Flashner <efraim@flashner.co.il> writes:
> 
> > On Wed, Apr 03, 2019 at 09:49:59PM +0200, Pierre Neidhardt wrote:
> >> Thank you Efraim, this is awesome!
> 
> I second this remark!  It's really helpful to see how you're doing it.
> Thank you for taking the time to share with us!
> 
> >> > tar xf $(guix build linux-libre -S)
> >> > cd linux-libre<tab>
> >> > touch .config
> >> > guix environment linux-libre
> >> >     make localmodconfig
> >> >
> >> > copy that into .config
> >> 
> >> Copy what into .config?
> >
> > the .config you just created at the root of the source tarball
> 
> I think there is some confusion here.  When I run "touch .config" and
> then run "make localmodconfig", the .config file is not modified.
> Perhaps you meant that we need to take the stdout from "make
> localmodconfig", save it, and copy it (after massaging it) back into the
> .config file?

Yeah, basically

> 
> > running make localconfig spits out a bunch of pseudo-error messages
> > about modules that are missing in .config. If you copy that whole
> > message, minus the lines starting with 'WARING', into .config and
> > then format it as UPERCASE_OPTION=(y|m) it gives the kernel a config
> > that is usable for it.
> >
> > so turning lines that look like:
> > module nouveau did not have configs CONFIG_DRM_NOUVEAU
> >
> > into lines that look like:
> > CONFIG_DRM_NOUVEAU=m
> 
> I read up a little on the various targets.  The documentation you
> mentioned was helpful, "make help" was helpful, the Makefile source was
> somewhat helpful, and the contents of
> ./scripts/kconfig/streamline_config.pl were helpful for understanding
> the intent of the "localmodconfig" target.
> 
> It sounds like the intended use of "localmodconfig" target is to modify
> an existing, non-empty .config file.  With that in mind, how does the
> procedure you describe compare to something like the following?
> 
> 1) Invoke "guix repl".  Then run this to determine the config that would
> normally be used to configure the default linux-libre kernel:
> 
>   scheme@(guix-user)> (assoc "kconfig" (package-native-inputs linux-libre))
>   $1 = ("kconfig" "/gnu/store/cwghfr06cadj2ss1ya6whgczpcba58z3-guix-module-union/share/guile/site/2.2/gnu/packages/aux-files/linux-libre/5.0-x86_64.conf")
> 
> 2) Copy that file out of the store into .config.
> 
> 3) Run "make olddefconfig" to non-interactively update the .config file.
> 
> 4) Run "make localmodconfig" to streamline the .config based on what's
> currently loaded.  I expected this target to also be non-interactive,
> but to my surprise it still asked me to choose what to set for a few
> options.
> 
> If you do it this way, you don't have to fix up formatting.  However,
> maybe it doesn't produce the same kind of configuration you had in mind.
> What do you think?
> 

This doesn't create a config that is as small as possible, which is what
I was looking for.

> > Currently this doesn't take into account the initrd.
> 
> Do you mean that there are modules or configuration which the initrd
> might need, which this procedure might accidentally disable?
> 

Yeah. I get different errors about missing ahci.ko or scsi.ko, so it's
going to take a bit more work.

> -- 
> Chris

I've started writing up a blog post about it (which I've attached), but
it doesn't work as-is.  More work incoming later I guess.


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Efraim Flashner   <efraim@flashner.co.il>   אפרים פלשנר
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title: Creating and using a custom Linux kernel on Guix System
date: 2019-04-04 00:00
author: Efraim Flashner
tags: kernel, customization
---

Guix is, at its core, a source based distribution with substitutes, and as such building packages from their source code is an expected part of regular package installations and upgrades.  Given this starting point, it makes sense that efforts are made to reduce the amount of time spent compiling packages, and recent changes and upgrades to the building and distribution of substitutes continues to be a topic of discussion within Guix.  One of the packages which I prefer to not build myself is the Linux-Libre kernel.  The kernel, while not requiring an overabundance of RAM to build, does take a very long time on my build machine (which my children argue is actually their Kodi computer), and I will often delay reconfiguring my laptop while I want for a substitute to be prepared by the official build farm.  The official kernel configuration, as is the case with many GNU/Linux distributions, errs on the side of inclusiveness, and this is really what causes the build to take such a long time when I build the package for myself.

The Linux kernel, however, can also just be described as a package installed on my machine, and as such can be customized just like any other package.  The procedure is a little bit different, although this is primarily due to the nature of how the package defintion is written.

The linux-libre kernel package definition is actually a procedure which creates a package.

```scheme
(define* (make-linux-libre version hash supported-systems
                           #:key
                           ;; A function that takes an arch and a variant.
                           ;; See kernel-config for an example.
                           (extra-version #f)
                           (configuration-file #f)
                           (defconfig "defconfig")
                           (extra-options %default-extra-linux-options)
                           (patches (list %boot-logo-patch)))
  ...)
```

The current linux-libre package is for the 5.0.x series, and is declared like this:

```scheme
(define-public linux-libre
  (make-linux-libre %linux-libre-version
                    %linux-libre-hash
                    '("x86_64-linux" "i686-linux" "armhf-linux" "aarch64-linux")
                    #:patches %linux-libre-5.0-patches
                    #:configuration-file kernel-config))
```

Any keys which are not assigned values inherit their default value from the make-linux-libre definition.  When comparing the two snippets above, you may notice that the code comment in the first doesn't actually refer to the extra-version keyword; it is actually for configuration-file.  Because of this, it is not actually easy to include a custom kernel configuration from the definition, but don't worry, there are other ways to work with what we do have.

There are two ways to create a kernel with a custom kernel configuration.  The first is to provide a standard '.config' file during the build process by including an actual '.config' file as a native-input to our custom kernel.  The following is a snippet from the custom 'configure phase of the make-linux-libre package definition:

```scheme
(let ((build  (assoc-ref %standard-phases 'build))
      (config (assoc-ref (or native-inputs inputs) "kconfig")))

  ;; Use a custom kernel configuration file or a default
  ;; configuration file.
  (if config
      (begin
        (copy-file config ".config")
        (chmod ".config" #o666))
      (invoke "make" ,defconfig))
```

Below is a sample kernel package for one of my computers:

```scheme
(define-public linux-libre-E2140
  (let ((base
          ((@@ (gnu packages linux) make-linux-libre)
           (@@ (gnu packages linux) %linux-libre-version)
           (@@ (gnu packages linux) %linux-libre-hash)
           '("x86_64-linux")
           #:extra-version "E2140"
           #:patches (@@ (gnu packages linux) %linux-libre-5.0-patches))))
    (package
      (inherit base)
      (native-inputs
       `(("kconfig" ,(local-file "E2140.config"))
         ,@(package-native-inputs base))))))
```

In the same directory as the file defining linux-libre-E2140 is a file named E2140.config, which is an actual kernel configuration file.  I left the defconfig keyword of make-linux-libre blank, so the only kernel configuration in the package is the one which I included as a native-input.

The second way to create a custom kernel is to pass a new value to the extra-options keyword of the make-linux-libre procedure.  The extra-options keyword works with another function defined right below it:

```scheme
(define %default-extra-linux-options
  `(;; https://lists.gnu.org/archive/html/guix-devel/2014-04/msg00039.html
   ("CONFIG_DEVPTS_MULTIPLE_INSTANCES" . #t)
   ;; Modules required for initrd:
   ("CONFIG_NET_9P" . m)
   ("CONFIG_NET_9P_VIRTIO" . m)
   ("CONFIG_VIRTIO_BLK" . m)
   ("CONFIG_VIRTIO_NET" . m)
   ("CONFIG_VIRTIO_PCI" . m)
   ("CONFIG_VIRTIO_BALLOON" . m)
   ("CONFIG_VIRTIO_MMIO" . m)
   ("CONFIG_FUSE_FS" . m)
   ("CONFIG_CIFS" . m)
   ("CONFIG_9P_FS" . m)))

(define (config->string options)
  (string-join (map (match-lambda
                      ((option . 'm)
                       (string-append option "=m"))
                      ((option . #t)
                       (string-append option "=y"))
                      ((option . #f)
                       (string-append option "=n")))
                    options)
               "\n"))
```

And in the custom configure script from the make-linux-libre package:

```scheme
;; Appending works even when the option wasn't in the
;; file.  The last one prevails if duplicated.
(let ((port (open-file ".config" "a"))
      (extra-configuration ,(config->string extra-options)))
  (display extra-configuration port)
  (close-port port))

(invoke "make" "oldconfig"))))
```

So by not providing a configuraion-file the '.config' starts blank, and then we write into it the collection of flags that we want.  Here's another custom kernel which I have:

```scheme
(define %macbook41-full-config
  (append %macbook41-config-options
          %filesystems
          %efi-support
          %emulation
          (@@ (gnu packages linux) %default-extra-linux-options)))

(define-public linux-libre-macbook41
  ((@@ (gnu packages linux) make-linux-libre) (@@ (gnu packages linux) %linux-libre-version)
                      (@@ (gnu packages linux) %linux-libre-hash)
                      '("x86_64-linux")
                      #:extra-version "macbook41"
                      #:patches (@@ (gnu packages linux) %linux-libre-5.0-patches)
                      #:extra-options %macbook41-config-options))
```

From the above example %filesystems is a collection of flags enabling different filesystem support, %efi-support enables EFI support and %emulation enables my x86_64-linux machine to act in 32-bit mode also.   %defalt-extra-linux-options are the ones quoted above, which had to be added in since I used replaced them in the extra-options keyword.

This all sounds like it should be doable, but how does one even know which modules are required for their system?  The two places I found most helpful to try to answer this question were the [Gentoo Handbook](https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Kernel), and the [documentation](https://www.kernel.org/doc/html/latest/admin-guide/README.html?highlight=localmodconfig) from the kernel itself.  From the kernel documentation, it seems that "make localmodconfig" is the command we want.

In order to actually run `make localmodconfig` we first need to get and unpack the kernel source code:

```shell
tar xf $(guix build linux-libre --source)
```

Once inside the directory containing the source code run `touch .config` to create an initial, empty '.config' to start with.  `make localmodconfig` works by seeing what you already have in '.config' and letting you know what you're missing.  If the file is blank then you're missing everything.  The next step is to run

```shell
make localmodconfig
```

and note the output.  Do note that the '.config' file is still empty.  The output generally contains two types of warnings.  The first start with "WARNING" and can actually be ignored in our case.  The second read:
```shell
module pcspkr did not have configs CONFIG_INPUT_PCSPKR
```

For each of these lines, copy the CONFIG_XXXX_XXXX portion into the '.config' in the directory, and append "=m", so in the end it looks like this:
```shell
CONFIG_INPUT_PCSPKR=m
CONFIG_VIRTIO=m
```

After copying all the configuration options, run `make localmodconfig` again to make sure that you don't have any output starting with "module".  After all of these machine specific modules there are a couple more left that are also needed.  CONFIG_MODULES is necessary so that you can build and load modules separately and not have everything built into the kernel.  CONFIG_BLK_DEV_SD is required for reading from hard drives.  It is possible that there are other modules which you will need.

The second way to setup the kernel configuration makes more use of Guix's features and allows you to share configuration segments between different kernels.  For example, all machines using EFI to boot have a number of EFI configuration flags that they need.  It is likely that all the kernels will share a list of filesystems to support.


#### About GNU Guix

[GNU Guix](https://www.gnu.org/software/guix) is a transactional package
manager and an advanced distribution of the GNU system that [respects
user
freedom](https://www.gnu.org/distros/free-system-distribution-guidelines.html).
Guix can be used on top of any system running the kernel Linux, or it
can be used as a standalone operating system distribution for i686,
x86_64, ARMv7, and AArch64 machines.

In addition to standard package management features, Guix supports
transactional upgrades and roll-backs, unprivileged package management,
per-user profiles, and garbage collection.  When used as a standalone
GNU/Linux distribution, Guix offers a declarative, stateless approach to
operating system configuration management.  Guix is highly customizable
and hackable through [Guile](https://www.gnu.org/software/guile)
programming interfaces and extensions to the
[Scheme](http://schemers.org) language.

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