# Notes on kernel development

## Low level kernel development

For kernel development it's not uncommon to need to monitor or break things
before the gfxconsole comes up.

To force-enable log output to the legacy serial console on an x64 machine, pass
"kernel.serial=legacy".  For other serial configurations, see the kernel.serial
docs in [kernel_cmdline.md](/docs/reference/kernel/kernel_cmdline.md).

To enable the early console before the graphical console comes up use the
``gfxconsole.early`` cmdline option. More information can be found in
[kernel_cmdline.md](/docs/reference/kernel/kernel_cmdline.md).
Enabling ``startup.keep-log-visible``will ensure that the kernel log stays
visible if the gfxconsole comes up after boot. To disable the gfxconsole
entirely you can disable the video driver it is binding to via ``driver.<driver
name>.disable``.
On a skylake system, all these options together would look something like:

```
$ tools/build-x86/bootserver build-x86/zircon.bin -- gfxconsole.early driver.intel-i915-display.disable
```

To directly output to the console rather than buffering it (useful in the event
of kernel freezes) you can enable ``ENABLE_KERNEL_LL_DEBUG`` in your build like so:

```
fx set ... --args='zircon_extra_args={kernel_extra_defines=["ENABLE_KERNEL_LL_DEBUG"]}'

```

There is also a kernel cmdline parameter kernel.bypass-debuglog, which can be set
to true to force output to the console instead of buffering it. The reason we have
both a compile switch and a cmdline parameter is to facilitate prints in the kernel
before cmdline is parsed to be forced to go to the console. The compile switch setting
overrides the cmdline parameter (if both are present). Note that both the compile switch
and the cmdline parameter have the side effect of disabling irq driven uart Tx.

## Changing the compiler optimization level of a module

You can override the default `-On` level for a module by defining in its
build arguments:

```
opt_level := <n>
```

## Requesting a backtrace

### From within a user process

For debugging purposes, the system crashlogger can print backtraces by
request. It requires modifying your source, but in the absence of a
debugger, or as a general builtin debug mechanism, this can be useful.

```
#include <lib/backtrace-request/backtrace-request.h>

void my_function() {
  backtrace_request();
}
```

When `backtrace\_request` is called, it causes an
exception used by debuggers for breakpoint handling.
If a debugger is not attached, the system crashlogger will
process the exception, print a backtrace, and then resume the thread.

### From a kernel thread

```
#include <kernel/thread.h>

void my_function() {
  thread_print_backtrace(get_current_thread(), __GET_FRAME(0));
}
```

## Exporting debug data during boot

To support testing the system during early boot, there is a mechanism to export
data files from the kernel to the /boot filesystem. To export a data file,
create a VMO, give it a name, and pass it to userboot with handle\_info of type
PA\_VMO\_DEBUG\_FILE (and argument 0). Then userboot will automatically pass it
through to devmgr, and devmgr will export the VMO as a file at the path

```
/boot/kernel/<name-of-vmo>
```

This mechanism is used by the entropy collector quality tests to export
relatively large (~1 Mbit) files full of random data.