zircon/docs/kernel_cmdline.md - fuchsia - Git at Google

 # Zircon Kernel Commandline Options

 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.

 ## 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.

 ## devmgr\.epoch=\<seconds\>

 Sets the initial offset (from the Unix epoch, in seconds) for the UTC clock.
 This is useful for platforms lacking an RTC, where the UTC offset would
 otherwise remain at 0.

 ## 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-debug

 If this option is set, the system prints out debugging when mexec, suspend,
 reboot, or power off did not finish in 10 seconds.

 ## 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>.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 DDK_LOG_... 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.tracing.enable=\<bool>

 Enable or disable support for tracing drivers.
 When enabled drivers may participate in [Fuchsia tracing](ddk/tracing.md).

 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.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.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.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.

 If set to "none", the kernel debug serial port will be 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.
 - 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`](#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 `/boot`, so it should not start with
 a `/` prefix.

 Note that this option does not work for executables that are linked with
 libraries other than libc and the dynamic linker.

 Example: `userboot=bin/core-tests`

 ## 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 with userboot=bin/core-tests in QEMU, this will cause the system to
 continually run tests and reboot.*

 ## userboot.shutdown

 If this option is set, userboot will attempt to power off the machine
 when the process it launches exits.

 ## vdso.soft_ticks=\<bool>

 If this option is set, the `zx_ticks_get` and `zx_ticks_per_second` system
 calls will use `zx_clock_get_monotonic()` in nanoseconds rather than
 hardware cycle counters in a hardware-based time unit.  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.

 # 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 scripts/run-zircon*

 Pass each option using -c, for example:
 ```
 ./scripts/run-zircon-x64 -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.
	# Zircon Kernel Commandline Options

	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.

	## 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.

	## devmgr\.epoch=\<seconds\>

	Sets the initial offset (from the Unix epoch, in seconds) for the UTC clock.
	This is useful for platforms lacking an RTC, where the UTC offset would
	otherwise remain at 0.

	## 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-debug

	If this option is set, the system prints out debugging when mexec, suspend,
	reboot, or power off did not finish in 10 seconds.

	## 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>.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 DDK_LOG_... 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.tracing.enable=\<bool>

	Enable or disable support for tracing drivers.
	When enabled drivers may participate in [Fuchsia tracing](ddk/tracing.md).

	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.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.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.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.

	If set to "none", the kernel debug serial port will be 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.
	- 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`](#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 `/boot`, so it should not start with
	a `/` prefix.

	Note that this option does not work for executables that are linked with
	libraries other than libc and the dynamic linker.

	Example: `userboot=bin/core-tests`

	## 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 with userboot=bin/core-tests in QEMU, this will cause the system to
	continually run tests and reboot.*

	## userboot.shutdown

	If this option is set, userboot will attempt to power off the machine
	when the process it launches exits.

	## vdso.soft_ticks=\<bool>

	If this option is set, the `zx_ticks_get` and `zx_ticks_per_second` system
	calls will use `zx_clock_get_monotonic()` in nanoseconds rather than
	hardware cycle counters in a hardware-based time unit. 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.

	# 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 scripts/run-zircon*

	Pass each option using -c, for example:
	```
	./scripts/run-zircon-x64 -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.