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.
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. 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='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.
You can override the default -On
level for a module by defining in its build arguments:
opt_level := <n>
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.
#include <kernel/thread.h> void my_function() { thread_print_backtrace(get_current_thread(), __GET_FRAME(0)); }
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.