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kernel: configs: update README

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Embellish the README a bit, and add some details about the
new fragment method.

Signed-off-by: Graham Whaley <graham.whaley@intel.com>
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Graham Whaley 2019-01-22 16:05:16 +00:00 committed by Julio Montes
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* [Kata Containers kernel config files](#kata-containers-kernel-config-files)
* [Types of config files](#types-of-config-files)
* [How to use config files](#how-to-use-config-files)
* [How to modify config files](#how-to-modify-config-files)
# Kata Containers kernel config files
This directory contains Linux Kernel config files used to configure Kata
Containers VM kernels.
## Types of config files
This directory holds config files for the Kata Linux Kernel in two forms:
- A tree of config file 'fragments' in the `fragments` sub-folder, that are
constructed into a complete config file using the kernel
`scripts/kconfig/merge_config.sh` script.
- As complete config files that can be used as-is.
Kernel config fragments are the preferred method of constructing `.config` files
to build Kata Containers kernels, due to their improved clarity and ease of maintenance
over single file monolithic `.config`s.
## How to use config files
config files must be copied in the kernel source code directory and renamed to `.config`
The recommended way to set up a kernel tree, populate it with a relevant `.config` file,
and build a kernel, is to use the [`build_kernel.sh`](../build-kernel.sh) script. For
example:
For example:
```bash
$ ./build-kernel.sh setup
```
```
cp x86_kata_kvm_4.14.x linux-4.14.22/.config
pushd linux-4.14.22
make ARCH=x86_64 -j4
```
The `build-kernel.sh` script understands both full and fragment based config files.
Run `./build-kernel.sh help` for more information.
## How to modify config files
Complete config files can be modified either with an editor, or preferably
using the kernel `Kconfig` configuration tools, for example:
```
cp x86_kata_kvm_4.14.x linux-4.14.22/.config
pushd linux-4.14.22
make menuconfig
popd
cp linux-4.14.22/.config x86_kata_kvm_4.14.x
$ cp x86_kata_kvm_4.14.x linux-4.14.22/.config
$ pushd linux-4.14.22
$ make menuconfig
$ popd
$ cp linux-4.14.22/.config x86_kata_kvm_4.14.x
```
Kernel fragments are best constructed using an editor. Tools such as `grep` and
`diff` can help find the differences between two config files to be placed
into a fragment.
If adding config entries for a new subsystem or feature, consider making a new
fragment with an appropriately descriptive name.
The fragment gathering tool perfoms some basic sanity checks, and the `build-kernel.sh` will
fail and report the error in the cases of:
- A duplicate `CONFIG` symbol appearing.
- A `CONFIG` symbol being in a fragment, but not appearing in the final .config
- which indicates that `CONFIG` variable is not a part of the kernel `Kconfig` setup, which
can indicate a typing mistake in the name of the symbol.
- A `CONFIG` symbol appearing in the fragments with multiple different values.