See //docs/gen/boot-options.md is now the source of truth.

The Zircon kernel receives a textual command line from the bootloader, which can be used to alter some behaviours of the system. Kernel command line parameters are in the form of option or option=value, separated by spaces, and may not contain spaces.

For boolean options, option=0, option=false, or option=off will disable the option. Any other form (option, option=true, option=wheee, etc) will enable it.

The kernel command line is passed from the kernel to the userboot process and the device manager, so some of the options described below apply to those userspace processes, not the kernel itself.

If keys are repeated, the last value takes precedence, that is, later settings override earlier ones.

Component Manager reads the file /boot/config/additional_boot_args (if it exists) at startup and imports name=value lines into its environment, augmenting or overriding the values from the kernel command line. Leading whitespace is ignored and lines starting with # are ignored. Whitespace is not allowed in names.

In order to specify options in the build, see this guide.

bootsvc.next=<bootfs path>

Controls what program is executed by bootsvc to continue the boot process. If this is not specified, the default next program will be used.

Arguments to the program can optionally be specified using a comma separator between the program and individual arguments. For example, 'bootsvc.next=bin/mybin,arg1,arg2'.

console.shell=<bool>

If this option is set to true driver_manager will launch the shell if kernel.shell has not already been launched. Defaults to false.

If this is false, it also disables the zircon.autorun.boot and zircon.autorun.system options.

driver.<name>.tests.enable=<bool>

Enable the unit tests for an individual driver. The unit tests will run before the driver binds any devices. If driver.tests.enable is true then this defaults to enabled, otherwise the default is disabled.

Note again that the name of the driver is the "Driver" argument to the ZIRCON_DRIVER macro. It is not, for example, the name of the device, which for some drivers is almost identical, except that the device may be named "foo-bar" whereas the driver name must use underscores, e.g., "foo_bar".

driver.tests.enable=<bool>

Enable the unit tests for all drivers. The unit tests will run before the drivers bind any devices. It's also possible to enable tests for an individual driver, see driver.\<name>.enable_tests. The default is disabled.

x64 specific values

On x64, some additional values are supported for configuring 8250-like UARTs:

• If set to legacy, the legacy COM1 interface is used.
• If set to acpi, the UART specified by the DBG2 ACPI entry on the system will be used, if available.
• A port-io UART can be specified using ioport,\<portno>,\<irq>.
• An MMIO UART can be specified using mmio,\<physaddr>,\<irq>.

For example, ioport,0x3f8,4 would describe the legacy COM1 interface.

All numbers may be in any base accepted by strtoul().

All other values are currently undefined.

ldso.trace

This option (disabled by default) turns on dynamic linker trace output. The output is in a form that is consumable by clients like Intel Processor Trace support.

zircon.autorun.boot=<command>

This option requests that command be run at boot.

Commands should be absolute paths starting at the root '/'.

Any + characters in command are treated as argument separators, allowing you to pass arguments to an executable.

This option is disabled if console.shell is false.

zircon.autorun.system=<command>

This option requests that command be run once the system partition is mounted. If there is no system partition, it will never be launched.

Commands should be absolute paths starting at the root '/'.

Any + characters in command are treated as argument separators, allowing you to pass arguments to an executable.

This option is disabled if console.shell is false.

zircon.system.pkgfs.cmd=<command>

This option requests that command be run once the blob partition is mounted. Any + characters in command are treated as argument separators, allowing you to pass arguments to an executable.

The executable and its dependencies (dynamic linker and shared libraries) are found in the blob filesystem. The executable path is command before the first +. The dynamic linker (PT_INTERP) and shared library (DT_NEEDED) name strings sent to the loader service are prefixed with lib/ to produce a path. Each such path is resolved to a blob ID (i.e. merkleroot in ASCII hex) using the zircon.system.pkgfs.file.path command line argument. In this way, /boot/config/additional_boot_args contains a fixed manifest of files used to start the process.

The new process receives a PA_USER0 channel handle at startup that will be used as the client filesystem handle mounted at /pkgfs. /pkgfs/system will also be mounted as /system.

zircon.system.pkgfs.file.path=<blobid>

Used with zircon.system.pkgfs.cmd, above.

netsvc.netboot=<bool>

If true, zircon will attempt to netboot into another instance of zircon upon booting.

More specifically, zircon will fetch a new zircon system from a bootserver on the local link and attempt to kexec into the new image, thereby replacing the currently running instance of zircon.

This setting implies zircon.system.disable-automount=true

netsvc.disable=<bool>

If set to true (default), netsvc is disabled.

netsvc.advertise=<bool>

If true, netsvc will seek a bootserver by sending netboot advertisements. Defaults to true.

netsvc.interface=<path>

This option instructs netsvc to use only the device whose topological path ends with the option's value, with any wildcard * characters matching any zero or more characters of the topological path. All other devices are ignored by netsvc. The topological path for a device can be determined from the shell by running the lsdev command on the device e.g. /dev/class/network/000 or /dev/class/ethernet/000).

This is useful for configuring network booting for a device with multiple ethernet ports, which may be enumerated in a non-deterministic order.

netsvc.all-features=<bool>

This option makes netsvc work normally and support all features. By default, netsvc starts in a minimal mode where only device discovery is supported.

userboot.next=<path>

This option instructs the userboot process (the first userspace process) to execute the specified binary within the bootfs, instead of following the normal userspace startup process (launching the device manager, etc).

It is useful for alternate boot modes (like a factory test or system unit tests).

The pathname used here is relative to userboot.root (below), if set, or else relative to the root of the BOOTFS (which later is ordinarily seen at /boot). It should not start with a / prefix.

If this executable uses PT_INTERP (i.e. the dynamic linker), the userboot process provides a loader service to resolve the PT_INTERP (dynamic linker) name and any shared library names it may request. That service simply looks in the lib/ directory (under userboot.root) in the BOOTFS.

Arguments to the next program can optionally be specified using a '+' separator between the program and individual arguments. The next program name will always be provided as the first argument.

Example: userboot.next=bin/core-tests+arg1+arg2=foo

userboot.root=<path>

This sets a "root" path prefix within the BOOTFS where the userboot.next path and the lib/ directory for the loader service will be found. By default, there is no prefix so paths are treated as exact relative paths from the root of the BOOTFS. e.g. with userboot.root=pkg/foo and userboot.next=bin/app, the names found in the BOOTFS will be pkg/foo/bin/app, pkg/foo/lib/ld.so.1, etc.

userboot.shutdown

If this option is set, userboot will attempt to power off the machine when the process it launches exits. Note if userboot.reboot is set then userboot.shutdown will be ignored.

zircon.nodename=<name>

Set the system nodename, as used by bootserver, loglistener, and the net{addr,cp,ls,runcmd} tools. If omitted, the system will generate a nodename from its MAC address. This cmdline is honored by GigaBoot and Zircon.

zircon.namegen=<num>

Set the system nodename generation style. If omitted or unknown, the system uses style 1. It has no effect if zircon.nodename is set. Older name generation styles may be removed in the future. This cmdline is honored by GigaBoot and Zircon.

Styles: - 0: Uses a four-word-name-style using based on the MAC address. - 1: fuchsia-0123-4567-89ab based on the MAC address.

zvb.current_slot=<_a|_b|_r>

Makes Fuchsia aware of the slot booted by the bootloader. Setting this also informs the paver that ABR is supported, and that it should update the ABR metadata.

zvb.boot-partition-uuid=<UUID>

An alternative to zvb.current_slot - makes Fuchsia aware of the slot booted by the bootloader by passing the UUID of the partition containing the Zircon kernel that was booted. Setting this also informs the paver that ABR is supported, and that it should update the ABR metadata.

console.device_topological_suffix=<path>

If this is set then console launcher will connect to the console device whose topological path matches this suffix. If not specified then console launcher will connect to /svc/console. Only has effect if kernel.shell=false.

Additional Gigaboot Command Line Options

bootloader.timeout=<num>

This option sets the boot timeout in the bootloader, with a default of 3 seconds. Set to zero to skip the boot menu.

bootloader.fbres=<w>x<h>

This option sets the framebuffer resolution. Use the bootloader menu to display available resolutions for the device.

Example: bootloader.fbres=640x480

bootloader.default=<network|local|zedboot>

This option sets the default boot device to netboot, use a local zircon.bin or to netboot via zedboot.

How to pass the command line to the kernel

in the emulator or Qemu, using ffx emu or fx qemu

Pass each option using -c, for example:

ffx emu start -c gfxconsole.font=18x32 -c gfxconsole.early=false

in GigaBoot20x6, when netbooting

Pass the kernel command line at the end, after a -- separator, for example:

bootserver zircon.bin bootfs.bin -- gfxconsole.font=18x32 gfxconsole.early=false

in GigaBoot20x6, when booting from USB flash

Create a text file named "cmdline" in the root of the USB flash drive's filesystem containing the command line.