Options common to all machines
aslr.disable=<bool>
Default: false
If this option is set, the system will not use Address Space Layout Randomization.
aslr.entropy_bits=<uint8_t>
Default: 0x1e
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.
kernel.cprng-reseed-require.hw-rng=<bool>
Default: false
When enabled and if HW RNG fails at reseeding, CPRNG panics.
kernel.cprng-reseed-require.jitterentropy=<bool>
Default: false
When enabled and if jitterentropy fails at reseeding, CPRNG panics.
kernel.cprng-seed-require.hw-rng=<bool>
Default: false
When enabled and if HW RNG fails at initial seeding, CPRNG panics.
kernel.cprng-disable.jitterentropy=<bool>
Default: false
Determines whether jitterentropy will be used as an entropy source (used for testing)
kernel.cprng-disable.hw-rng=<bool>
Default: false
Determines whether HW RNG will be used as an entropy source (used for testing)
kernel.cprng-seed-require.jitterentropy=<bool>
Default: false
When enabled and if jitterentropy fails initial seeding, CPRNG panics.
kernel.cprng-seed-require.cmdline=<bool>
Default: false
When enabled and if you do not provide entropy input from the kernel command line, CPRNG panics.
kernel.entropy-mixin=<hexadecimal>
Provides entropy to be mixed into the kernel's CPRNG. The value must be a string of lowercase hexadecimal digits.
The original value will be scrubbed from memory as soon as possible and will be redacted from all diagnostic output.
kernel.jitterentropy.bs=<uint32_t>
Default: 0x40
Sets the "memory block size" parameter for jitterentropy. 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=<uint32_t>
Default: 0x200
Sets the "memory block count" parameter for jitterentropy. 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=<uint32_t>
Default: 0x20
Sets the "memory loops" parameter for jitterentropy. 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=<uint32_t>
Default: 0x1
Sets the "LFSR loops" parameter for jitterentropy. 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>
Default: true
When true, 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=<uint64_t>
Default: 0xbb8
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.
When kernel.lockup-detector.heartbeat-period-ms is 0, critical section lockup detection is disabled.
kernel.lockup-detector.critical-section-fatal-threshold-ms=<uint64_t>
Default: 0x2710
When a CPU remains in a designated critical section for longer than this
threshold, a crashlog will be generated and the system will reboot, indicating a
reboot reason of SOFTWARE_WATCHDOG as it does.
See also k lockup status and
lockup detector.
When 0, critical section crashlog generation and reboot is disabled.
When kernel.lockup-detector.heartbeat-period-ms is 0, critical section lockup detection is disabled.
kernel.lockup-detector.heartbeat-period-ms=<uint64_t>
Default: 0x3e8
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=<uint64_t>
Default: 0xbb8
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 period, 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.lockup-detector.heartbeat-age-fatal-threshold-ms=<uint64_t>
Default: 0x2710
The maximum age of a CPU's last heartbeat before it is considered to be locked
up, triggering generation of a crashlog indicating a reboot reason of
SOFTWARE_WATCHDOG followed by a reboot.
See also lockup detector.
When 0, heartbeat crashlog generation and reboot is disabled.
kernel.lockup-detector.diagnostic-query-timeout-ms=<uint64_t>
Default: 0x64
The maximum amount of time to spend trying to get diagnostic data from an unresponsive CPU before giving up.
When 0, querying for diagnostic data is disabled.
kernel.oom.behavior=[reboot | jobkill]
Default: reboot
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 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()), waits some period, and then reboots the
system. The length of the wait period is set by the
kernel.oom.reboot-timeout-ms boot option.
kernel.oom.reboot-timeout-ms=<uint32_t>
Default: 0xc350
This option sets the amount of time the kernel will wait before rebooting the
system when it encounters an out-of-memory (OOM) situation. This option is
only relevant when kernel.oom.behavior is set to reboot.
kernel.mexec-force-high-ramdisk=<bool>
Default: false
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).
kernel.mexec-pci-shutdown=<bool>
Default: true
If false, this option leaves PCI devices running when calling mexec.
kernel.oom.enable=<bool>
Default: true
This option 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 mem dump.
kernel.oom.outofmemory-mb=<uint64_t>
Default: 0x32
This option 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=<uint64_t>
Default: 0x96
This option 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=<uint64_t>
Default: 0x12c
This option 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=<uint64_t>
Default: 0x1
This option 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>
Default: false
This option 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.oom.evict-continuous=<bool>
Default: false
This option configures kernel eviction to run continually in the background to try and keep the system out of memory pressure, as opposed to triggering one-shot eviction only at memory pressure level transitions.
kernel.oom.hysteresis-seconds=<uint64_t>
Default: 0xa
This option specifies the hysteresis interval (in seconds) between memory pressure state transitions. Note that hysteresis is only applicable for transitions from a state with less free memory to a state with more free memory; transitions in the opposite direction are not delayed.
kernel.oom.imminent-oom-delta-mb=<uint64_t>
Default: 0xa
This option specifies the delta (in MB) above the out-of-memory threshold at which an imminent-out-of-memory event will be signaled. This signal is intended to be used for capturing diagnostic memory information close to the OOM, since capturing state exactly at the OOM might not be possible.
For example, if kernel.oom.outofmemory-mb is set to 50 and kernel.oom.imminent-oom-delta-mb
is set to 20, an imminent-out-of-memory event will be signaled at 70MB (i.e. 50MB + 20MB)
free memory, while out-of-memory will be signaled at 50MB free memory.
kernel.oom.trigger-on-alloc-failure=<bool>
Default: true
This option controls whether potentially user-visible PMM allocation failures due to running out of memory trigger an OOM response.
kernel.phys.next=<string>
Default: physboot
The name of the file within the kernel storage filesystem physload will load.
kernel.phys.verbose=<bool>
Default: true
This controls the degree of logging to the serial console in the kernel's early boot phase; if false, only error-related logging will take place.
One utility of this option is for benchmarking: synchronous, single-threaded UART writing can be relatively costly (10 chars/ms) to the entire time spent in physboot and it is desirable to exclude this sort of work from holistic time measurements.
kernel.phys.backtrace-max=<uint32_t>
Default: 0x40
When there is a crash in the kernel's early boot phase, it can print out backtraces on the serial console; it prints both a backtrace based on frame pointers and, when built to use shadow call stacks also a parallel backtrace based on the shadow call stack. Each backtrace will print no more than this many frames. (Most backtraces will end with the outermost frame before hitting the limit.) Setting the limit to zero prints unlimited frames, which for the frame-pointers backtrace can get into an infinite loop with some bugs.
kernel.phys.print-stack-max=<uint32_t>
Default: 0x400
When there is a crash in the kernel's early boot phase, it can print out stack contents on the serial console. This is the maximum size (in bytes) of stack memory that will be dumped; the actual amount dumped depends on stack depth. Each 16 bytes of stack is printed on one line using up to 75 characters.
kernel.ppb.borrow-in-supplypages=<bool>
Default: false
This controls whether zx_pager_supply_pages can borrow loaned pages. If true, zx_pager_supply_pages will copy supplied page contents into borrowed pages, if any loaned pages are available and the supplied pages aren't already loaned, else zx_pager_supply_pages will install the supplied pages into the VMO. If false, zx_pager_supply_pages will install the supplied pages into the VMO (regardless of whether they are already loaned or not).
kernel.ppb.borrow-on-mru=<bool>
Default: false
This controls whether non-loaned pages will be replaced with loaned pages (if any loaned pages are available) when non-loaned pages are moved to the MRU queue. This moving is done lazily under normal circumstances, but near OOM as eviction proceeds, any pages that should be in the MRU queue are moved to the MRU queue.
kernel.ppb.loan=<bool>
Default: false
This controls whether ZX_VMO_OP_DECOMMIT is enabled on a contiguous VMO. If true, decommit on a contiguous VMO can work and return ZX_OK. The pages are loaned to the rest of the system for potential borrowing. If false, decommit will return ZX_ERR_NOT_SUPPORTED.
kernel.ppb.replace-on-unloan=<bool>
Default: false
This controls what happens when a loaned page is re-committed to its original contiguous VMO. If false, the page is evicted from where it is loaned from, requiring that content to be faulted back in. If true, the contents are copied to a new page that is swapped in to replace the loaned page.
kernel.render-dlog-to-crashlog=<bool>
Default: false
When set to true, as much of the recent kernel debuglog as will fit will be appended to the generated crashlog during a kernel panic to assist in debugging.
kernel.serial=[none | legacy | qemu | <type>,<base>,<irq>]
Default: none
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.
The configuration string format is as follows: For MMIO UART: "kernel.serial=UART_MODEL,MMIO_ADDR,IRQ,FLAGS" For PIO UART: "kernel.serial=UART_MODEL,IOPORT,IRQ"
kernel.vdso.always_use_next=<bool>
Default: false
If this option is set, the kernel will supply userspace with the "next" vDSO rather than the "stable" vDSO as the default vDSO.
vdso.ticks_get_force_syscall=<bool>
Default: false
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.
vdso.clock_get_force_syscall=<bool>
Default: false
If this option is set, the zx_clock_get_monotonic and zx_clock_get_boot vDSO calls will be
forced to be a true syscall, instead of simply performing a transformation of the tick
counter in user-mode.
kernel.userpager.overtime_wait_seconds=<uint64_t>
Default: 0x14
This option 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=<uint64_t>
Default: 0x12c
This option 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.heap-max-size-mb=<uint64_t>
Default: 0x800
This option configures the maximum size of the heap. Only has effect if kernel has been compiled to use a virtual heap.
kernel.heap.alloc_fill_threshold=<uint64_t>
Default: 0x0
When set, the kernel heap will fill allocations below this size (in bytes).
kernel.bufferchain.reserve-pages=<uint64_t>
Default: 0x20
Specifies the number of pages per CPU to reserve for buffer chain allocations (channel messages). Higher values reduce contention on the PMM when the system is under load at the cost of using more memory when the system is idle.
TODO(https://fxbug.dev/42147481): Determine an upper bound for this value to prevent consuming too much memory.
kernel.bypass-debuglog=<bool>
Default: false
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.debug_uart_poll=<bool>
Default: false
If true, will periodically poll UART and forwards its contents into the console.
kernel.enable-debugging-syscalls=<bool>
Default: false
When disabled, certain debugging-related syscalls will fail with
ZX_ERR_NOT_SUPPORTED. These are:
zx_debug_send_command()zx_ktrace_control()zx_ktrace_init()zx_ktrace_read()zx_ktrace_write()zx_mtrace_control()zx_object_get_property()withZX_PROP_PROCESS_HW_TRACE_CONTEXT_IDzx_process_write_memory()zx_system_mexec()zx_system_mexec_payload_get()zx_thread_write_state()(When using theZX_THREAD_STATE_DEBUG_REGSkind.)zx_vmo_op_range()withZX_VMO_OP_CACHE_INVALIDATE
kernel.enable-serial-syscalls=[false | true | output-only]
Default: false
When false, 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-test.src=[hw_rng | jitterentropy]
Default: hw_rng
When running an entropy collector quality test, use the provided entropy source.
This option is ignored unless the kernel was built with ENABLE_ENTROPY_COLLECTOR_TEST=1.
kernel.entropy-test.len=<uint64_t>
Default: 0x100000
When running an entropy collector quality test, collect the provided number of bytes.
The maximum value can be increased by defining ENTROPY_COLLECTOR_TEST_MAXLEN as such value.
kernel.force-watchdog-disabled=<bool>
Default: false
When set, 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>
Default: false
If this option is set, 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.txtandzircon.elffiles 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.
ktrace.bufsize=<uint32_t>
Default: 0x20
This option specifies the number of megabytes allocated for ktrace records.
ktrace.grpmask=<uint32_t>
Default: 0x0
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.
kernel.memory-limit-dbg=<bool>
Default: true
This option enables verbose logging from the memory limit library.
kernel.memory-limit-mb=<uint64_t>
Default: 0x0
This option sets an upper-bound in megabytes for the system memory. If set to zero, then no upper-bound is set.
For example, choosing a low enough value would allow a user simulating a system with less physical memory than it actually has.
kernel.page-scanner.enable-eviction=<bool>
Default: true
When set, allows the page 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.page-table-eviction-policy=[always | never | on_request]
Default: always
Sets the reclamation policy for user page tables that are not accessed.
When on_request, only performs eviction on request, such as in response to a
low memory scenario.
When never, page tables are never evicted.
When always, Unused page tables are evicted periodically. The period can be controlled by
kernel.page-scanner.page-table-eviction-period.
kernel.page-scanner.page-table-eviction-period-ms=<uint32_t>
Default: 0x2710
Sets the rate, in milliseconds, that page tables will be scanned. Any page tables not used between two successive scans are candidates for eviction.
This option only has an effect if kernel.page-scanner.page-table-eviction-policy=always.
kernel.page-scanner.min-aging-interval-ms=<uint32_t>
Default: 0x7d0
Sets the minimum time, in milliseconds, between successive aging events. Higher values here will provide a more stable active set with less chance of thrashing like behavior and less time spent harvesting page access information. Lower values will allow for smaller active sets, increasing opportunities for eviction.
This value should be less than or equal to kernel.page-scanner.max-aging-interval-ms.
kernel.page-scanner.max-aging-interval-ms=<uint32_t>
Default: 0x3a98
Sets the maximum time, in milliseconds, between successive aging events. This time is the potential worst case coarseness of page age information, and higher values can result in not having sufficient age information to perform eviction if system behavior rapidly changes. Lower values will cause pages to lose fidelity information by accumulating in the oldest bucket increasing the chance that when eviction happens a sub-optimal page is chosen.
This value should be greater than or equal to kernel.page-scanner.min-aging-interval-ms.
kernel.page-scanner.accessed-scan-interval-ms=<uint32_t>
Default: 0xbb8
Sets the time, in milliseconds, between harvesting page access information. Lower values provide greater age fidelity and will improve accuracy of page reclamation choices at the expense of increased CPU time spent harvesting.
There is no benefit to setting this to be lower than the
kernel.page-scanner.min-aging-interval-ms.
kernel.page-scanner.active-ratio-multiplier=<uint32_t>
Default: 0x2
Controls the allowable ratio of active pages, compared to inactive pages, before aging is triggered.
The ratio is represented as a multiplier to simplify the kernel algorithm which is
should_age = active_page_count * active_ratio_multiplier > inactive_page_count. Higher multipliers
will result in more frequent aging events and hence less pages being in the active set. A multiplier
of 0 will disable aging based on the active page ratio.
The active ratio only triggers aging in the period between the
kernel.page-scanner.min-aging-interval-ms and the kernel.page-scanner.max-aging-interval-ms,
and as such has no effect if the intervals are equal.
kernel.page-scanner.start-at-boot=<bool>
Default: true
This option 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=<uint64_t>
Default: 0x4e20
This option 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.
This value was chosen to consume, in the worst case, 5% CPU on a lower-end arm device. Individual configurations may wish to tune this higher (or lower) as needed.
kernel.page-scanner.lru-action=[none | evict_only | compress_only | evict_and_compress]
Default: none
Sets any reclamation action to take on old pages in the LRU queue. These actions occur regardless of
memory pressure. If the none action is specified then pages will not be reclaimed just because
they are old.
Supported actions are:
- none
- evict_only
- compress_only
- evict_and_compress
kernel.compression.random-debug-compress=<bool>
Default: false
Enables a testing option that causes the VM to random immediately compress pages, regardless of their age. This is intended to facilitate testing of the compression paths without needing to wait for pages to age and put the system into a low memory state. It is an error to enable this option if the kernel is not built with debugging assertions enabled, or if a compression and storage strategy are not set.
kernel.compression.strategy=[none | lz4]
Default: none
Supported compression strategies are:
- none
- lz4
This option selects the desired compression strategy to be used when a page needs to be compressed.
If none is set then kernel.compression.storage-strategy must also be none. Selecting none
effectively disables compression.
kernel.compression.storage-strategy=[none | tri_page]
Default: none
Supported compression storage strategies are:
- none
- tri_page
This option selects the desired storage strategy to be used for storing data that has been
compressed. If none is set then kernel.compression.strategy must also be none.
kernel.compression.threshold=<uint32_t>
Default: 0x46
This option controls the compression threshold that must be achieved for a compressed page to be stored. Pages that do not achieve this threshold will continue to be stored uncompressed instead.
The option is a percentage, with a value of 100 meaning a page just needs to compress to at least its original size, and a value of 0 meaning that page would need to compress to a zero size.
Valid values are between 1 and 100, inclusive.
kernel.compression.lz4.acceleration=<uint32_t>
Default: 0xb
This option controls the acceleration factor provided to the LZ4 compression implementation. Refer to the current LZ4 implementation for how this value will be interpreted.
kernel.compression.at_memory_pressure=<bool>
Default: false
This option controls whether page compression should be performed in response to memory pressure.
For this option to have any effect kernel.page-scanner.enable-eviction needs to be enabled and both kernel.compression.strategy and kernel.compression.storage-strategy need to be set.
kernel.compression.reclaim_anonymous=<bool>
Default: false
This option controls whether anonymous pages are placed in the reclaimable page queues and have associated age information. Enabling this adds a small overhead to anonymous pages for age tracking, but is required for options like "kernel.compression.at_memory_pressure" to have any effect.
kernel.compression.reclaim_zero_forks=<bool>
Default: false
This option is similar to "kernel.compression.reclaim_anonymous" but applies to the zero forks of anonymous pages. Enabling this makes the "kernel.page-scanner.zero-page-scans-per-second" option have no effect, and it is an error to enable this without enabling "kernel.compression.reclaim_anonymous".
kernel.pmm-checker.action=[oops | panic]
Default: oops
This option specifies which action is taken when the PMM checker detects
corruption. Values must be one of:
* oops
- A non-fatal kernel OOPS will be emitted when corruption is detected.
* panic
- A fatal kernel panic will occur when corruption is detected.
kernel.pmm-checker.enable=[true | false | auto]
Default: false
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. Values must be one of:
* true
- Checker is always enabled.
* false
- Checker is never enabled.
* auto
- Kernel makes a decision to enable checker based on the environment and its performance
characteristics. Running under a hypervisor is one factor taken into consideration.
See also "k pmm checker".
kernel.pmm-checker.fill-size=<uint64_t>
Default: 0x1000
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.
kernel.pmm.alloc-random-should-wait=<bool>
Default: false
Enables a testing option that causes the PMM to randomly fail wait-able page allocations with ZX_ERR_SHOULD_WAIT, regardless of the current memory level. This is intended to facilitate testing of the waiting paths without needing to put the system into a low memory state. It is an error to enable this option if the kernel is not built with debugging assertions enabled.
kernel.portobserver.reserve-pages=<uint64_t>
Default: 0x8
Specifies the number of pages per CPU to reserve for port observer (async wait) allocations. Higher values reduce contention on the PMM when the system is under load at the cost of using more memory when the system is idle.
kernel.portpacket.reserve-pages=<uint64_t>
Default: 0x1
Specifies the number of pages per CPU to reserve for port packet (port_queue) allocations. Higher values reduce contention on the PMM when the system is under load at the cost of using more memory when the system is idle.
kernel.root-job.behavior=[halt | reboot | bootloader | recovery | shutdown]
Default: reboot
This option specifies what action the kernel should take when the root job is either terminated, or has no jobs and no processes.
When halt, will halt the system.
When reboot, will reboot the system.
When bootloader, will reboot the system into the bootloader.
When recovery, will reboot the system into the recovery partition.
When shutdown, will shutdown 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.shell=<bool>
Default: false
Tells the kernel to start its own shell on the kernel console instead of starting userspace.
kernel.shell.script=<string>
Tells the kernel to run a canned script of kernel.shell commands. If
kernel.shell is also specified, the kernel shell command prompt comes
after the script completes unless the script shuts down the system.
Since whitespace on the kernel command line separates different boot options,
+ characters in the script are replaced with spaces to allow for commands
with arguments, and ; characters can separate commands in the script.
kernel.smp.ht=<bool>
Default: true
This option specifies whether the HyperThreading (HT) logical CPUs should be enabled or not.
kernel.test.ram.reserve=<std::optional>
Specifies a range of physical RAM to be reserved for testing purposes. This should be written as just SIZE (an integer byte quantity, which should be page-aligned) but will be read back as SIZE,ADDRESS after the kernel assigns an address in early boot.
kernel.port.max-observers=<uint64_t>
Default: 0xc350
Specifies the maximum number of observers any single port may have. When this limit is reached, a Zircon exception is raised in the process that crosses the limit. This value should be high enough that well behaved programs will not hit the limit, but low enough to terminate misbehaving programs before they impact the system.
kernel.select=<string>
Default: zircon
The name of the kernel package to boot from the STORAGE_KERNEL item in the ZBI.
kernel.scheduler.prefer-little-cpus=<bool>
Default: false
When searching for a CPU on which to place a task, prefer little cores over big cores. Enabling this option trades off improved performance in favor of reduced power consumption.
DEPRECATED - This option is scheduled to be removed in an upcoming release. Do not take any critical dependencies on it.
kernel.ubsan.action=[oops | panic]
Default: panic
When the kernel is instrumented with UndefinedBehaviorSanitizer, problems
it detects are reported on the serial console. These can be fatal or not.
Values must be one of:
* oops
- Detected undefined behavior causes a non-fatal kernel OOPS.
* panic
- Detected undefined behavior causes a fatal kernel panic.
Options available only on arm64 machines
kernel.arm64.disable_spec_mitigations=<bool>
Default: false
If set, disables all speculative execution information leak mitigations.
If unset, the per-mitigation defaults will be used.
kernel.arm64.event-stream.enable=<bool>
Default: true
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.
kernel.arm64.event-stream.freq-hz=<uint32_t>
Default: 0x2710
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.
kernel.arm64.debug.dap-rom-soc=<string>
If set, tries to initialize the dap debug aperture at a hard coded address for the particular system on chip. Currently accepted values are amlogic-t931g, amlogic-s905d2, amlogic-s905d3g, and amlogic-a311d.
kernel.smp.maxcpus=<uint32_t>
Default: 0x10
This option caps the number of CPUs to initialize. It cannot be greater than SMP_MAX_CPUS for a specific architecture.
kernel.phys.psci-reset=[disabled | shutdown | reboot | reboot-bootloader | reboot-recovery]
Default: reboot
This option determines what kind of PSCI reset operation (if any) the early boot kernel will use if it needs to panic and crash. If this is "disabled", the machine may enter an infinite loop on panic.
kernel.arm64.phys.mmu=<bool>
Default: true
This enables use of the MMU and caches during the kernel's early boot phase.
kernel.arm64.enable-asid=<bool>
Default: true
This enables use of ASIDs. True by default if the underlying hardware supports 16-bit ASIDs.
kernel.arm64.alternate-vbar=[none | auto | arch3 | arch1 | psci | smccc10]
Default: auto
This selects the alternate exception vector implementation used to work around
CPU-specific issues on entry to EL1 from EL0. Values can be:
* none - No mitigations performed in early EL0 exception paths.
* auto (default) - Select SMCCC function identifier based on availability
reported by firmware via SMCCC / PSCI interfaces. Then each individual CPU
queries the firmware for whether the workaround is needed at all. If the
firmware is not SMCCC >= 1.1 that reports it supports the
SMCCC_ARCH_WORKAROUND_3 and/or SMCCC_ARCH_WORKAROUND_1 function identifiers,
then this has the same effect as none. If the firmware does support it,
then each CPU either uses the firmware workaround or not, according to what
the firmware says is needed on that CPU. Either way, alternative spectre
mitigations, that are either redundant with the firmware workaround or not
needed if the firmware workaround is not needed, are disabled.
* arch3 - Always use SMCCC_ARCH_WORKAROUND_3 on every CPU. This may have
unpredictable effects if the firmware does not support SMCCC 1.1 or does not
support SMCCC_ARCH_WORKAROUND_3. This makes every CPU do what auto will
select for each individual CPU that the firmware says should use it, when
the firmware supports the function. This also disables alternative spectre
mitigations that are redundant with SMCCC_ARCH_WORKAROUND_3.
* arch1 - Always use SMCCC_ARCH_WORKAROUND_1 on every CPU. This may have
unpredictable effects if the firmware does not support SMCCC 1.1 or does not
support SMCCC_ARCH_WORKAROUND_1. This makes every CPU do what auto will
select for each individual CPU that the firmware says should use it, when
the firmware supports the "1" function but not the "3" function. This also
disables alternative spectre mitigations that are redundant with
SMCCC_ARCH_WORKAROUND_1.
* psci - Always use PSCI_VERSION with SMCCC 1.1 calling conventions, on
every CPU. This may have unpredictable effects if the firmware does not
support SMCCC 1.1. Currently auto never selects this behavior for any
CPU.
* smccc10 - Always use PSCI_VERSION with SMCCC 1.0 calling conventions, on
every CPU. This is safe on all systems, though has it more overhead than
SMCCC 1.1 options and may not be necessary on all CPUs. Currently auto
never selects this behavior for any CPU.
As on SoCs with heterogeneous cores, a different selection should be made for
each individual CPU, values other than auto should only be used in testing.
kernel.arm64.force-pct=<bool>
Default: false
By default, the kernel will select the ARM virtual counter (VCT) to use as the time reference for the system. It will only choose the physical counter (PCT) if it was given an IRQ for the physical timer hardware, but not the virtual timer hardware. When the force-pct option is set, the kernel will always choose to use the PCT as the time reference, even if a VCT IRQ is offered in the ZBI. Please note, setting force-pct on a target which has no defined PCT IRQ will result in a panic during early boot.
Options available only on riscv64 machines
kernel.smp.maxcpus=<uint32_t>
Default: 0x10
This option caps the number of CPUs to initialize. It cannot be greater than SMP_MAX_CPUS for a specific architecture.
kernel.riscv64.enable-asid=<bool>
Default: true
This enables use of ASIDs. True by default if the underlying hardware supports 16-bit ASIDs.
kernel.riscv64.phys.mmu=<bool>
Default: true
This enables use of the MMU in the kernel's early boot phase.
Options available only on x86 machines
kernel.x86.disable_spec_mitigations=<bool>
Default: false
If set, disables all speculative execution information leak mitigations.
If unset, the per-mitigation defaults will be used.
kernel.x86.hwp=<bool>
Default: true
This settings enables HWP (hardware P-states) on supported chips. This feature lets Intel CPUs automatically scale their own clock speed.
kernel.x86.hwp_policy=[bios-specified | performance | balanced | power-save | stable-performance]
Default: bios-specified
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: Use the power/performance tradeoff policy specified in firmware/BIOS settings. If no policy is available, falls back tobalanced.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.md_clear_on_user_return=<bool>
Default: true
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.
kernel.x86.spec_store_bypass_disable=<bool>
Default: false
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.turbo=<bool>
Default: true
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.
kernel.x86.enable_pcid=<bool>
Default: true
This option controls whether PCIDs are used, if there is sufficient CPU support. If this option is enabled and there is not sufficient CPU support, then this option has no effect.
kernel.smp.maxcpus=<uint32_t>
Default: 0x20
This option caps the number of CPUs to initialize. It cannot be greater than SMP_MAX_CPUS for a specific architecture.
kernel.wallclock=[auto | tsc | pit | hpet]
Default: auto
This option can be used to force the selection of a particular wall clock on pc builds.