1.. _kernelparameters: 2 3The kernel's command-line parameters 4==================================== 5 6The following is a consolidated list of the kernel parameters as implemented 7by the __setup(), early_param(), core_param() and module_param() macros 8and sorted into English Dictionary order (defined as ignoring all 9punctuation and sorting digits before letters in a case insensitive 10manner), and with descriptions where known. 11 12The kernel parses parameters from the kernel command line up to "``--``"; 13if it doesn't recognize a parameter and it doesn't contain a '.', the 14parameter gets passed to init: parameters with '=' go into init's 15environment, others are passed as command line arguments to init. 16Everything after "``--``" is passed as an argument to init. 17 18Module parameters can be specified in two ways: via the kernel command 19line with a module name prefix, or via modprobe, e.g.:: 20 21 (kernel command line) usbcore.blinkenlights=1 22 (modprobe command line) modprobe usbcore blinkenlights=1 23 24Parameters for modules which are built into the kernel need to be 25specified on the kernel command line. modprobe looks through the 26kernel command line (/proc/cmdline) and collects module parameters 27when it loads a module, so the kernel command line can be used for 28loadable modules too. 29 30Hyphens (dashes) and underscores are equivalent in parameter names, so:: 31 32 log_buf_len=1M print-fatal-signals=1 33 34can also be entered as:: 35 36 log-buf-len=1M print_fatal_signals=1 37 38Double-quotes can be used to protect spaces in values, e.g.:: 39 40 param="spaces in here" 41 42cpu lists: 43---------- 44 45Some kernel parameters take a list of CPUs as a value, e.g. isolcpus, 46nohz_full, irqaffinity, rcu_nocbs. The format of this list is: 47 48 <cpu number>,...,<cpu number> 49 50or 51 52 <cpu number>-<cpu number> 53 (must be a positive range in ascending order) 54 55or a mixture 56 57<cpu number>,...,<cpu number>-<cpu number> 58 59Note that for the special case of a range one can split the range into equal 60sized groups and for each group use some amount from the beginning of that 61group: 62 63 <cpu number>-<cpu number>:<used size>/<group size> 64 65For example one can add to the command line following parameter: 66 67 isolcpus=1,2,10-20,100-2000:2/25 68 69where the final item represents CPUs 100,101,125,126,150,151,... 70 71The value "N" can be used to represent the numerically last CPU on the system, 72i.e "foo_cpus=16-N" would be equivalent to "16-31" on a 32 core system. 73 74Keep in mind that "N" is dynamic, so if system changes cause the bitmap width 75to change, such as less cores in the CPU list, then N and any ranges using N 76will also change. Use the same on a small 4 core system, and "16-N" becomes 77"16-3" and now the same boot input will be flagged as invalid (start > end). 78 79The special case-tolerant group name "all" has a meaning of selecting all CPUs, 80so that "nohz_full=all" is the equivalent of "nohz_full=0-N". 81 82The semantics of "N" and "all" is supported on a level of bitmaps and holds for 83all users of bitmap_parselist(). 84 85This document may not be entirely up to date and comprehensive. The command 86"modinfo -p ${modulename}" shows a current list of all parameters of a loadable 87module. Loadable modules, after being loaded into the running kernel, also 88reveal their parameters in /sys/module/${modulename}/parameters/. Some of these 89parameters may be changed at runtime by the command 90``echo -n ${value} > /sys/module/${modulename}/parameters/${parm}``. 91 92The parameters listed below are only valid if certain kernel build options 93were enabled and if respective hardware is present. This list should be kept 94in alphabetical order. The text in square brackets at the beginning 95of each description states the restrictions within which a parameter 96is applicable:: 97 98 ACPI ACPI support is enabled. 99 AGP AGP (Accelerated Graphics Port) is enabled. 100 ALSA ALSA sound support is enabled. 101 APIC APIC support is enabled. 102 APM Advanced Power Management support is enabled. 103 APPARMOR AppArmor support is enabled. 104 ARM ARM architecture is enabled. 105 ARM64 ARM64 architecture is enabled. 106 AX25 Appropriate AX.25 support is enabled. 107 CLK Common clock infrastructure is enabled. 108 CMA Contiguous Memory Area support is enabled. 109 DRM Direct Rendering Management support is enabled. 110 DYNAMIC_DEBUG Build in debug messages and enable them at runtime 111 EARLY Parameter processed too early to be embedded in initrd. 112 EDD BIOS Enhanced Disk Drive Services (EDD) is enabled 113 EFI EFI Partitioning (GPT) is enabled 114 EVM Extended Verification Module 115 FB The frame buffer device is enabled. 116 FTRACE Function tracing enabled. 117 GCOV GCOV profiling is enabled. 118 HIBERNATION HIBERNATION is enabled. 119 HW Appropriate hardware is enabled. 120 HYPER_V HYPERV support is enabled. 121 IA-64 IA-64 architecture is enabled. 122 IMA Integrity measurement architecture is enabled. 123 IP_PNP IP DHCP, BOOTP, or RARP is enabled. 124 IPV6 IPv6 support is enabled. 125 ISAPNP ISA PnP code is enabled. 126 ISDN Appropriate ISDN support is enabled. 127 ISOL CPU Isolation is enabled. 128 JOY Appropriate joystick support is enabled. 129 KGDB Kernel debugger support is enabled. 130 KVM Kernel Virtual Machine support is enabled. 131 LIBATA Libata driver is enabled 132 LOONGARCH LoongArch architecture is enabled. 133 LOOP Loopback device support is enabled. 134 LP Printer support is enabled. 135 M68k M68k architecture is enabled. 136 These options have more detailed description inside of 137 Documentation/arch/m68k/kernel-options.rst. 138 MDA MDA console support is enabled. 139 MIPS MIPS architecture is enabled. 140 MOUSE Appropriate mouse support is enabled. 141 MSI Message Signaled Interrupts (PCI). 142 MTD MTD (Memory Technology Device) support is enabled. 143 NET Appropriate network support is enabled. 144 NFS Appropriate NFS support is enabled. 145 NUMA NUMA support is enabled. 146 OF Devicetree is enabled. 147 PARISC The PA-RISC architecture is enabled. 148 PCI PCI bus support is enabled. 149 PCIE PCI Express support is enabled. 150 PCMCIA The PCMCIA subsystem is enabled. 151 PNP Plug & Play support is enabled. 152 PPC PowerPC architecture is enabled. 153 PPT Parallel port support is enabled. 154 PS2 Appropriate PS/2 support is enabled. 155 PV_OPS A paravirtualized kernel is enabled. 156 RAM RAM disk support is enabled. 157 RDT Intel Resource Director Technology. 158 RISCV RISCV architecture is enabled. 159 S390 S390 architecture is enabled. 160 SCSI Appropriate SCSI support is enabled. 161 A lot of drivers have their options described inside 162 the Documentation/scsi/ sub-directory. 163 SECURITY Different security models are enabled. 164 SELINUX SELinux support is enabled. 165 SERIAL Serial support is enabled. 166 SH SuperH architecture is enabled. 167 SMP The kernel is an SMP kernel. 168 SPARC Sparc architecture is enabled. 169 SUSPEND System suspend states are enabled. 170 SWSUSP Software suspend (hibernation) is enabled. 171 TPM TPM drivers are enabled. 172 UMS USB Mass Storage support is enabled. 173 USB USB support is enabled. 174 USBHID USB Human Interface Device support is enabled. 175 V4L Video For Linux support is enabled. 176 VGA The VGA console has been enabled. 177 VMMIO Driver for memory mapped virtio devices is enabled. 178 VT Virtual terminal support is enabled. 179 WDT Watchdog support is enabled. 180 X86-32 X86-32, aka i386 architecture is enabled. 181 X86-64 X86-64 architecture is enabled. 182 More X86-64 boot options can be found in 183 Documentation/arch/x86/x86_64/boot-options.rst. 184 X86 Either 32-bit or 64-bit x86 (same as X86-32+X86-64) 185 X86_UV SGI UV support is enabled. 186 XEN Xen support is enabled 187 XTENSA xtensa architecture is enabled. 188 189In addition, the following text indicates that the option:: 190 191 BOOT Is a boot loader parameter. 192 BUGS= Relates to possible processor bugs on the said processor. 193 KNL Is a kernel start-up parameter. 194 195Parameters denoted with BOOT are actually interpreted by the boot 196loader, and have no meaning to the kernel directly. 197Do not modify the syntax of boot loader parameters without extreme 198need or coordination with <Documentation/arch/x86/boot.rst>. 199 200There are also arch-specific kernel-parameters not documented here. 201See for example <Documentation/arch/x86/x86_64/boot-options.rst>. 202 203Note that ALL kernel parameters listed below are CASE SENSITIVE, and that 204a trailing = on the name of any parameter states that that parameter will 205be entered as an environment variable, whereas its absence indicates that 206it will appear as a kernel argument readable via /proc/cmdline by programs 207running once the system is up. 208 209The number of kernel parameters is not limited, but the length of the 210complete command line (parameters including spaces etc.) is limited to 211a fixed number of characters. This limit depends on the architecture 212and is between 256 and 4096 characters. It is defined in the file 213./include/uapi/asm-generic/setup.h as COMMAND_LINE_SIZE. 214 215Finally, the [KMG] suffix is commonly described after a number of kernel 216parameter values. These 'K', 'M', and 'G' letters represent the _binary_ 217multipliers 'Kilo', 'Mega', and 'Giga', equaling 2^10, 2^20, and 2^30 218bytes respectively. Such letter suffixes can also be entirely omitted: 219 220.. include:: kernel-parameters.txt 221 :literal: 222