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.

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 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.

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 30.

blobfs.cache-eviction-policy=<policy>

Controls blobfs' eviction strategy for pager-backed blobs with no open handles or VMO clones. If unset, an internally defined system default is used.

Blobs which are not pager-backed are not affected by this knob.

The following values are supported:

• NEVER_EVICT: Nodes are never evicted. It is recommended to enable kernel page eviction (kernel.page-scanner.enable-user-pager-eviction) in this case, as otherwise blobfs will indefinitely retain all data pages in memory.
• EVICT_IMMEDIATELY: Nodes are evicted as soon as they have no open handles or VMO clones. They will need to be loaded from disk again on next access.

blobfs.write-compression-algorithm=<algorithm>

The compression algorithm that blobfs should use for writing blobs at runtime. If unset, an internally defined system default is used.

The following values are supported:

• ZSTD_CHUNKED
• UNCOMPRESSED

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.

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.

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.log-to-debuglog

If this option is set, devmgr and all drivers will output logs to debuglog, as opposed to syslog.

devmgr.verbose

If this option is set, devmgr will enable 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 minumum log severity for a driver. The textual constants "error", "warn", "info", "trace", "spew", may be used, and they map to the corresponding bits in DDK_LOG_... in ddk/debug.h. If an unknown value is passed, the minimum log severity will be set to "spew". The default minimum log severity for a driver is "info".

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.sysmem.protected_memory_size=<num>

Overrides the board-driver-specified size for sysmem's default protected memory pool. Value is in bytes.

driver.sysmem.protected_memory_size=<num>

Overrides the board-driver-specified size for sysmem's contiguous memory pool. Value is in bytes.

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.

driver.iommu.enable=<bool>

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

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.

kernel.arm64.event-stream.enable=<bool>

When enabled, each ARM cpu will enable an event stream generator, which per-cpu sets the hidden event flag at a particular rate. This has the effect of kicking cpus out of any WFE states they may be sitting in. The default is false.

kernel.arm64.event-stream.freq-hz=<uint32>

If the event stream is enabled, specifies the frequency at which it will attempt to run. The resolution is limited, so the driver will only be able to pick the nearest power of 2 from the cpu timer counter. The default is 10000.

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.cprng-reseed-require.hw-rng=<bool>

When enabled and if HW RNG fails at reseeding, CPRNG panics. Defaults to false.

kernel.cprng-reseed-require.jitterentropy=<bool>

When enabled and if jitterentropy fails at reseeding, CPRNG panics. Defaults to false.

kernel.cprng-seed-require.hw-rng=<bool>

When enabled and if HW RNG fails at initial seeding, CPRNG panics. Defaults to false.

kernel.cprng-seed-require.jitterentropy=<bool>

When enabled and if jitterentrop fails initial seeding, CPRNG panics. Defaults to false.

kernel.cprng-seed-require.cmdline=<bool>

When enabled and if you do not provide entropy input from the kernel command line, CPRNG panics. Defaults to false.

kernel.enable-debugging-syscalls=<bool>

When disabled, certain debugging-related syscalls will fail with ZX_ERR_NOT_SUPPORTED. Defaults to false (debugging syscalls disabled). These are: - zx_debug_send_command() - zx_ktrace_control() - zx_ktrace_init() - zx_ktrace_read() - zx_mtrace_control() - zx_process_write_memory() - zx_system_mexec() - zx_system_mexec_payload_get()

kernel.enable-serial-syscalls=<string>

Can be one of three values: - false - true - output-only

When false (the default), both zx_debug_read() and zx_debug_write() will fail with ZX_ERR_NOT_SUPPORTED.

When output-only, zx_debug_read() will fail with ZX_ERR_NOT_SUPPORTED, but zx_debug_write() will work normally.

When true, both will work normally.

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.force-watchdog-disabled=<bool>

If this option is set (disabled by default), the system will attempt to disable any hardware watchdog timer armed and passed by the bootloader as soon as it possibly can in the boot sequence, presuming that the bootloader provides enough information to know how to disable the WDT at all.

kernel.halt-on-panic=<bool>

If this option is set (disabled by default), the system will halt on a kernel panic instead of rebooting. To enable halt-on-panic, pass the kernel command line argument kernel.halt-on-panic=true.

Since the kernel can't reliably draw to a framebuffer when the GPU is enabled, the system will reboot by default if the kernel crashes or panics.

If the kernel crashes and the system reboots, the log from the kernel panic will appear at /boot/log/last-panic.txt, suitable for viewing, downloading, etc.

Please attach your last-panic.txt and zircon.elf files to any kernel panic bugs you file.

If there's a last-panic.txt, that indicates that this is the first successful boot since a kernel panic occurred.

It is not "sticky" -- if you reboot cleanly, it will be gone, and if you crash again it will be replaced.

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.lockup-detector.critical-section-threshold-ms=<num>

When a CPU remains in a designated critical section for longer than this threshold, a KERNEL OOPS will be emitted.

See also k lockup status and lockup detector.

When 0, critical section lockup detection is disabled.

kernel.lockup-detector.heartbeat-period-ms=<num>

How frequently a secondary CPU should emit a heartbeat via kernel timer. This value should be large enough to not impact system performance, but should be smaller than the heartbeat age threshold. 1000 is a reasonable value.

See also lockup detector.

When 0, heartbeat detection is disabled.

kernel.lockup-detector.heartbeat-age-threshold-ms=<num>

The maximum age of a secondary CPU's last heartbeat before it is considered to be locked up. This value should be larger than the heartbeat peroid, but small enough so as to not miss short-lived lockup events. 3000 is a reasonable value.

See also lockup detector.

When 0, heartbeat detection is disabled.

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 out-of-memory (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 out-of-memory 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 or reboots the system (per kernel.oom.behavior), when the PMM has less than kernel.oom.outofmemory-mb free memory.

An OOM can be manually triggered by the command k pmm oom, which will cause free memory to fall below the kernel.oom.outofmemory-mb threshold. An allocation rate can be provided with k pmm oom <rate>, where <rate> is in MB. This will cause the specified amount of memory to be allocated every second, which can be useful for observing memory pressure state transitions.

Refer to kernel.oom.outofmemory-mb, kernel.oom.critical-mb, kernel.oom.warning-mb, and zx_system_get_event() for further details on memory pressure state transitions.

The current memory availability state can be queried with the command k pmm mem_avail_state info.

kernel.oom.outofmemory-mb=<num>

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

kernel.oom.critical-mb=<num>

This option (150 MB by default) specifies the free-memory threshold at which the out-of-memory (OOM) thread will trigger a critical memory pressure event, signaling that processes should free up memory.

kernel.oom.warning-mb=<num>

This option (300 MB by default) specifies the free-memory threshold at which the out-of-memory (OOM) thread will trigger a warning memory pressure event, signaling that processes should slow down memory allocations.

kernel.oom.debounce-mb=<num>

This option (1 MB by default) specifies the memory debounce value used when computing the memory pressure state based on the free-memory thresholds (kernel.oom.outofmemory-mb, kernel.oom.critical-mb and kernel.oom.warning-mb). Transitions between memory availability states are debounced by not leaving a state until the amount of free memory is at least kernel.oom.debounce-mb outside of that state.

For example, consider the case where kernel.oom.critical-mb is set to 100 MB and kernel.oom.debounce-mb set to 5 MB. If we currently have 90 MB of free memory on the system, i.e. we're in the Critical state, free memory will have to increase to at least 105 MB (100 MB + 5 MB) for the state to change from Critical to Warning.

kernel.oom.evict-at-warning=<bool>

This option, false by default, triggers eviction of file pages at the Warning pressure state, in addition to the default behavior, which is to evict at the Critical and OOM states.

kernel.root-job.behavior=<string>

This option specifies what action the kernel should take when the root job is either terminated, or has no jobs and no processes. This can take one of the following values:

• halt -- Halt the system
• reboot -- Reboot the system (the default)
• bootloader -- Reboot into the bootloader
• recovery -- Reboot into the recovery partition
• shutdown -- Shut down the system

kernel.root-job.notice=<string>

The option allows a notice to be printed when the root job is either: terminated, or has no jobs and no processes.

kernel.page-scanner.start-at-boot=<bool>

This option (true by default) causes the kernels active memory scanner to be initially enabled on startup. You can also enable and disable it using the kernel console. If you disable the scanner, you can have additional system predictability since it removes time based and background memory eviction.

Every action the scanner performs can be individually configured and disabled. If all actions are disabled then enabling the scanner has no effect.

kernel.page-scanner.zero-page-scans-per-second=<num>

This option, 20000 by default, configures the maximal number of candidate pages the zero page scanner will consider every second.

Setting to zero means no zero page scanning will occur. This can provide additional system predictability for benchmarking or other workloads.

The page scanner must be running for this option to have any effect. It can be enabled at boot with the kernel.page-scanner.start-at-boot option.

kernel.page-scanner.enable-user-pager-eviction=<bool>

This option, true by default, allows for the scanner to evict user pager backed pages. Eviction can reduce memory usage and prevent out of memory scenarios, but removes some timing predictability from system behavior.

kernel.page-scanner.promote-no-clones=<bool>

This option, false by default, allows the scanner to evict first the pages owned by pager backed VMOs with no clones. VMOs with no clones are meant to approximate inactive VMOs. Evicting inactive pages first is expected to free up memory that remains free for longer, and can be more effective at relieving memory pressure.

kernel.userpager.overtime_wait_seconds=<num>

This option, 20 by default, configures how long a user pager fault may block before being considered overtime and printing an information message to the debuglog and continuing to wait. A value of 0 indicates a wait is never considered to be overtime.

kernel.userpager.overtime_timeout_seconds=<num>

This option, 300 by default, configures how long a user pager fault may block before being aborted. For a hardware page fault, the faulting thread will terminate with a fatal page fault exception. For a software page fault triggered by a syscall, the syscall will fail with ZX_ERR_TIMED_OUT. A value of 0 indicates a page fault is never aborted due to a time out.

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.hwp=<bool>

This settings enables HWP (hardware P-states) on supported chips. This feature lets Intel CPUs automatically scale their own clock speed. Defaults to true.

kernel.x86.hwp_policy=<string>

Set a power/performance tradeoff policy of the CPU. x86 CPUs with HWP (hardware P-state) support can be configured to autonomusly scale their frequency to favour different policies.

Currently supported policies are:

• bios-specified: (default) Use the power/performance tradeoff policy specified in firmware/BIOS settings. If no policy is available, falls back to balanced.
• performance: Maximise performance.
• balanced: Balance performance / power savings.
• power-save: Reduce power usage, at the cost of lower performance.
• stable-performance: Use settings that keep system performance consistent. This may be useful for benchmarking, for example, where keeping performance predictable is more important than maximising performance.

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.x86.md_clear_on_user_return=<bool>

MDS (Microarchitectural Data Sampling) is a family of speculative execution information leak bugs that allow the contents of recent loads or stores to be inferred by hostile code, regardless of privilege level (CVE-2019-11091, CVE-2018-12126, CVE-2018-12130, CVE-2018-12127). For example, this could allow user code to read recent kernel loads/stores.

To avoid this bug, it is required that all microarchitectural structures that could leak data be flushed on trust level transitions. Also, it is important that trust levels do not concurrently execute on a single physical processor core.

This option controls whether microarchitectual structures are flushed on the kernel to user exit path, if possible. It may have a negative performance impact.

• If set to true (the default), structures are flushed if the processor is vulnerable.
• If set to false, no flush is executed on structures.

This option only affects x86 systems.

kernel.x86.turbo=<bool>

Turbo Boost or Core Performance Boost are mechanisms that allow processors to dynamically vary their performance at runtime based on available thermal and electrical budget. This may provide improved interactive performance at the cost of performance variability. Some workloads may benefit from disabling Turbo; if this command line flag is set to false, turbo is disabled for all CPUs in the system.

Defaults on.

This option only affects x86 systems.

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.
• 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.

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".

kernel.pmm-checker.enable=<bool>

This controls whether the PMM's use-after-free checker is enabled. The PMM checker can be expensive and is intended for use in debug and development builds. See also "k pmm checker". Defaults to false.

kernel.pmm-checker.fill-size=<num>

This option specifies how many bytes of each free page is filled or checked when the PMM's use-after-free checker is enabled. Valid values are multiples of 8, between 8 and PAGE_SIZE, inclusive. Defaults to PAGE_SIZE.

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.

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 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.

This option is disabled if console.shell is false.

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.

zircon.system.wait-for-data=<bool>

This option initializes pkgfs and appmgr only after a persistent data partition appears.

By default, this option is set to true.

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 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.

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=<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.

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.

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 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 fx emu or fx qemu

Pass each option using -c, for example:

fx qemu -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.