The Zircon kernel receives a textual commandline from the bootloader, which can be used to alter some behaviours of the system. Kernel commandline 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 commandline 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.

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

aslr.disable

If this option is set, the system will not use Address Space Layout Randomization.

aslr.entropy_bits=<num>

For address spaces that use ASLR this controls the number of bits of entropy in the randomization. Higher entropy results in a sparser address space and uses more memory for page tables. Valid values range from 0-36, with default being 36.

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'.

clock.backstop=<seconds>

Sets the initial offset (from the Unix epoch, in seconds) for the UTC clock. The clock will be set by the device coordinator at boot time, and then later, if an RTC is present, the RTC clock will be sanitized to at least this time.

devmgr.require-system=<bool>

Instructs the devmgr that a /system volume is required. Without this, devmgr assumes this is a standalone Zircon build and not a full Fuchsia system.

devmgr.suspend-timeout-fallback

If this option is set, the system invokes kernel fallback to reboot or poweroff the device when the operation did not finish in 10 seconds.

devmgr.devhost.asan

This option must be set if any drivers not included directly in /boot are built with -fsanitize=address. If there are -fsanitize=address drivers in /boot, then all -fsanitize=address drivers will be supported regardless of this option. If this option is not set and there are no such drivers in /boot, then drivers built with -fsanitize=address cannot be loaded and will be rejected.

devmgr.devhost.strict-linking

If this option is set, devmgr will only allow libasync-default.so, libdriver.so, and libfdio.so to be dynamically linked into a devhost. This prevents drivers from dynamically linking with libraries that they should not. All other libraries should be statically linked into a driver.

devmgr.verbose

Turn on verbose logging.

driver.<name>.compatibility-tests-enable

If this option is set, devmgr will run compatibility tests for the driver. zircon_driver_info, and can be found as the first argument to the ZIRCON_DRIVER_BEGIN macro.

driver.<name>.compatibility-tests-wait-time

This timeout lets you configure the wait time in milliseconds for each of bind/unbind/suspend hooks to complete in compatibility tests. zircon_driver_info, and can be found as the first argument to the ZIRCON_DRIVER_BEGIN macro.

driver.<name>.disable

Disables the driver with the given name. The driver name comes from the zircon_driver_info, and can be found as the first argument to the ZIRCON_DRIVER_BEGIN macro.

Example: driver.usb_audio.disable

driver.<name>.log=<flags>

Set the log flags for a driver. Flags are one or more comma-separated values which must be preceded by a "+" (in which case that flag is enabled) or a "-" (in which case that flag is disabled). The textual constants "error", "warn", "info", "trace", "spew", "debug1", "debug2", "debug3", and "debug4" may be used, and they map to the corresponding bits in DDKLOG... in ddk/debug.h The default log flags for a driver is "error", "warn", and "info".

Individual drivers may define their own log flags beyond the eight mentioned above.

Example: driver.usb_audio.log=-error,+info,+0x1000

Note again that the name of the driver is the "Driver" argument to the ZIRCON_DRIVER_BEGIN 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.<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_BEGIN 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.

driver.tracing.enable=<bool>

Enable or disable support for tracing drivers. When enabled drivers may participate in Fuchsia tracing.

Implementation-wise, what this option does is tell each devhost whether to register as "trace provider".

The default is enabled. This options exists to provide a quick fallback should a problem arise.

gfxconsole.early=<bool>

This option (disabled by default) requests that the kernel start a graphics console during early boot (if possible), to display kernel debug print messages while the system is starting. When userspace starts up, a usermode graphics console driver takes over.

The early kernel console can be slow on some platforms, so if it is not needed for debugging it may speed up boot to disable it.

gfxconsole.font=<name>

This option asks the graphics console to use a specific font. Currently only "9x16" (the default) and "18x32" (a double-size font) are supported.

iommu.enable=<bool>

This option (disabled by default) allows the system to use a hardware IOMMU if present.

kernel.bypass-debuglog=<bool>

When enabled, forces 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.

kernel.enable-debugging-syscalls=<bool>

When disabled, certain debugging-related syscalls will fail with ZX_ERR_NOT_SUPPORTED. Defaults to false (debugging syscalls disabled).

kernel.entropy-mixin=<hex>

Provides entropy to be mixed into the kernel's CPRNG.

kernel.entropy-test.len=<len>

When running an entropy collector quality test, collect the provided number of bytes. Defaults to the maximum value ENTROPY_COLLECTOR_TEST_MAXLEN.

The default value for the compile-time constant ENTROPY_COLLECTOR_TEST_MAXLEN is 1MiB.

kernel.entropy-test.src=<source>

When running an entropy collector quality test, use the provided entropy source. Currently recognized sources: hw_rng, jitterentropy. This option is ignored unless the kernel was built with ENABLE_ENTROPY_COLLECTOR_TEST=1.

kernel.halt-on-panic=<bool>

If this option is set (disabled by default), the system will halt on a kernel panic instead of rebooting.

kernel.jitterentropy.bs=<num>

Sets the "memory block size" parameter for jitterentropy (the default is 64). When jitterentropy is performing memory operations (to increase variation in CPU timing), the memory will be accessed in blocks of this size.

kernel.jitterentropy.bc=<num>

Sets the "memory block count" parameter for jitterentropy (the default is 512). When jitterentropy is performing memory operations (to increase variation in CPU timing), this controls how many blocks (of size kernel.jitterentropy.bs) are accessed.

kernel.jitterentropy.ml=<num>

Sets the "memory loops" parameter for jitterentropy (the default is 32). When jitterentropy is performing memory operations (to increase variation in CPU timing), this controls how many times the memory access routine is repeated. This parameter is only used when kernel.jitterentropy.raw is true. If the value of this parameter is 0 or if kernel.jitterentropy.raw is false, then jitterentropy chooses the number of loops is a random-ish way.

kernel.jitterentropy.ll=<num>

Sets the "LFSR loops" parameter for jitterentropy (the default is 1). When jitterentropy is performing CPU-intensive LFSR operations (to increase variation in CPU timing), this controls how many times the LFSR routine is repeated. This parameter is only used when kernel.jitterentropy.raw is true. If the value of this parameter is 0 or if kernel.jitterentropy.raw is false, then jitterentropy chooses the number of loops is a random-ish way.

kernel.jitterentropy.raw=<bool>

When true (the default), the jitterentropy entropy collector will return raw, unprocessed samples. When false, the raw samples will be processed by jitterentropy, producing output data that looks closer to uniformly random. Note that even when set to false, the CPRNG will re-process the samples, so the processing inside of jitterentropy is somewhat redundant.

kernel.memory-limit-dbg=<bool>

This option enables verbose logging from the memory limit library.

kernel.memory-limit-mb=<num>

This option tells the kernel to limit system memory to the MB value specified by 'num'. Using this effectively allows a user to simulate the system having less physical memory than physically present.

kernel.mexec-force-high-ramdisk=<bool>

This option is intended for test use only. When set to true it forces the mexec syscall to place the ramdisk for the following kernel in high memory (64-bit address space, >= 4GiB offset). The default value is false.

kernel.oom.behavior=<string>

This option can be used to configure the behavior of the kernel when encountering an OOM situation. Valid values are jobkill, and reboot. If unset or set to an invalid value, defaults to reboot.

If set to jobkill, when encountering OOM, the kernel attempts to kill jobs that have the ZX_PROP_JOB_KILL_ON_OOM bit set to recover memory.

If set to reboot, when encountering OOM, the kernel signals an event (see zx_system_get_event()), delays briefly, and then reboots the system.

kernel.oom.enable=<bool>

This option (true by default) turns on the out-of-memory (OOM) kernel thread, which kills processes when the PMM has less than kernel.oom.redline_mb free memory, sleeping for kernel.oom.sleep_sec between checks.

The OOM thread can be manually started/stopped at runtime with the k oom start and k oom stop commands, and k oom info will show the current state.

See k oom for a list of all OOM kernel commands.

kernel.oom.redline-mb=<num>

This option (50 MB by default) specifies the free-memory threshold at which the out-of-memory (OOM) thread will trigger a low-memory event and begin killing processes.

The k oom info command will show the current value of this and other parameters.

kernel.oom.sleep-sec=<num>

This option (1 second by default) specifies how long the out-of-memory (OOM) kernel thread should sleep between checks.

The k oom info command will show the current value of this and other parameters.

kernel.x86.disable_spec_mitigations=<bool>

If set, disable all speculative execution information leak mitigations.

If clear, the per-mitigation defaults will be used.

This option only affects x86 systems.

kernel.x86.pti.enable=<int>

Page table isolation configures user page tables to not have kernel text or data mapped. This may impact performance negatively. This is a mitigation for Meltdown (AKA CVE-2017-5754).

• If set to 1, this force-enables page table isolation.
• If set to 0, this force-disables page table isolation. This may be insecure.
• If set to 2 or unset (the default), this enables page table isolation on CPUs vulnerable to Meltdown.

This option only affects x86 systems.

kernel.x86.spec_store_bypass_disable=<bool>

Spec-store-bypass (Spectre V4) is a speculative execution information leak vulnerability that affects many Intel and AMD x86 CPUs. It targets memory disambiguation hardware to infer the contents of recent stores. The attack only affects same-privilege-level, intra-process data.

This command line option controls whether a mitigation is enabled. The mitigation has negative performance impacts.

• If true, the mitigation is enabled on CPUs that need it.
• If false (the default), the mitigation is not enabled.

kernel.mexec-pci-shutdown=<bool>

If false, this option leaves PCI devices running when calling mexec. Defaults to true.

kernel.serial=<string>

This controls what serial port is used. If provided, it overrides the serial port described by the system's bootdata. The kernel debug serial port is a reserved resource and may not be used outside of the kernel.

If set to "none", the kernel debug serial port will be disabled and will not be reserved, allowing the default serial port to be used outside the kernel.

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.
• 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.

kernel.shell=<bool>

This option tells the kernel to start its own shell on the kernel console instead of a userspace sh.

kernel.smp.maxcpus=<num>

This option caps the number of CPUs to initialize. It cannot be greater than SMP_MAX_CPUS for a specific architecture.

kernel.smp.ht=<bool>

This option can be used to disable the initialization of hyperthread logical CPUs. Defaults to true.

kernel.wallclock=<name>

This option can be used to force the selection of a particular wall clock. It only is used on pc builds. Options are "tsc", "hpet", and "pit".

ktrace.bufsize

This option specifies the size of the buffer for ktrace records, in megabytes. The default is 32MB.

ktrace.grpmask

This option specifies what ktrace records are emitted. The value is a bitmask of KTRACE_GRP_* values from zircon/ktrace.h. Hex values may be specified as 0xNNN.

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, after devmgr starts up.

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

zircon.autorun.system=<command>

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

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

zircon.system.disable-automount=<bool>

This option prevents the fshost from auto-mounting any disk filesystems (/system, /data, etc), which can be useful for certain low level test setups. It is false by default. It is implied by netsvc.netboot=true

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/devmgr 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. The command is expected to start serving on this channel and then signal its process handle with ZX_USER_SIGNAL_0. Then /pkgfs/system will be mounted as /system.

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

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

zircon.system.volume=<arg>

This option specifies where to find the "/system" volume.

It may be set to: "any", in which case the first volume of the appropriate type will be used. "local" in which the first volume that's non-removable of the appropriate type will be used. "none" (default) which avoids mounting anything.

A "/system" ramdisk provided by bootdata always supersedes this option.

zircon.system.filesystem-check=<bool>

This option requests that filesystems automatically mounted by the system are pre-verified using a filesystem consistency checker before being mounted.

By default, this option is set to false.

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.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 ethernet device at the given topological path. All other ethernet devices are ignored by netsvc. The topological path for a device can be determined from the shell by running the lsdev command on the ethernet class device (e.g., /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.

userboot=<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.

Example: userboot=bin/core-tests

userboot.root=<path>

This sets a "root" path prefix within the BOOTFS where the userboot 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=bin/app, the names found in the BOOTFS will be pkg/foo/bin/app, pkg/foo/lib/ld.so.1, etc.

userboot.reboot

If this option is set, userboot will attempt to reboot the machine after waiting 3 seconds when the process it launches exits.

If running a "ZBI test" image in QEMU, this will cause the system to continually run tests and reboot. For QEMU, userboot.shutdown is usually preferable.

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.

vdso.ticks_get_force_syscall=<bool>

If this option is set, the zx_ticks_get vDSO call will be forced to be a true syscall, even if the hardware cycle counter registers are accessible from user-mode. Defaults to false.

vdso.clock_get_monotonic_force_syscall=<bool>

If this option is set, the zx_clock_get_monotonic vDSO call will be forced to be a true syscall, instead of simply performing a transformation of the tick counter in user-mode. Defaults to false.

virtcon.disable

Do not launch the virtual console service if this option is present.

virtcon.hide-on-boot

If this option is present, the virtual console will not take ownership of any displays until the user switches to it with a device control key combination.

virtcon.keep-log-visible

If this option is present, the virtual console service will keep the debug log (vc0) visible instead of switching to the first shell (vc1) at startup.

virtcon.keymap=<name>

Specify the keymap for the virtual console. "qwerty" and "dvorak" are supported.

virtcon.font=<name>

Specify the font for the virtual console. "9x16" and "18x32" are supported.

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 human-readable nodename from its MAC address. This cmdline is honored by GigaBoot and Zircon.

console.path=<path>

Specify console device path. If not specified device manager will open /dev/misc/console. Only has effect if kernel.shell=false.

console.is_virtio=<bool>

Specify if the device given with console.path is a virtio-console device. Defaults to false. This is needed as a workaround due to drivers not being able to implement fuchsia.io.File themselves.

Additional Gigaboot Commandline 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 commandline to the kernel

in Qemu, using fx run

Pass each option using -c, for example:

fx run -c gfxconsole.font=18x32 -c gfxconsole.early=false

in GigaBoot20x6, when netbooting

Pass the kernel commandline 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.

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