According to FreeBSD Handbook, there are two ways of building a custom kernel: The Traditional Way and The New Way.
Either way, we should create a custom kernel configuration file, Then we use this custom kernel configuration for building the new kernel:
$ cd /usr/src
$ cp sys/amd64/config/GENERIC sys/amd64/config/CUSTOM
For example, we can enable kernel debuggings enabled in our custom configuration:
$ cat >> sys/amd64/config/CUSTOM <EOF
makeoptions -g
options KDB
options DDB
EOF
Now, to compile the new kernel with our custom configuration:
make -j 16 buildkernel KERNCONF=CUSTOM
make -j 16 installkernel KERNCONF=CUSTOM
If you're experiencing modifying kernel and trying to avoid re- compiling the kernel every time you have a small change in the source code, use the following command:
make -j 16 buildkernel KERNCONF=CUSTOM -DKERNFAST
Tips: Remember to remove the multiple make jobs -j flags
if you have errors in compiling the source to easily spot the
error message:
make buildkernel KERNCONF=CUSTOM -DKERNFAST