Chapter 5. Important changes to external kernel parameters

arm64.nompam=

[ARM64] Unconditionally disable Memory Partitioning And Monitoring support.

indirect_target_selection=

[X86,Intel] Mitigation control for Indirect Target Selection (ITS) bug in Intel CPUs. Updated microcode is also required for a fix in IBPB.

pcie.notph

[PCIE] If the PCIE_TPH kernel configuration parameter is enabled, this kernel boot option can be used to disable PCIe TLP Processing Hints support system-wide.

rcutree.csd_lock_suppress_rcu_stall=

[KNL] Do only a one-line RCU CPU stall warning when there is an ongoing too-long CSD-lock wait.

rcuscale.kfree_by_call_rcu=

[KNL] In kernels built with CONFIG_RCU_LAZY=y, test call_rcu() instead of kfree_rcu().

rcuscale.kfree_mult=

[KNL] Instead of allocating an object of size kfree_obj, allocate one of kfree_mult * sizeof(kfree_obj). Defaults to 1.

rcuscale.scale_type=

[KNL] Specify the RCU implementation to test.

rcutorture.stall_cpu_repeat=

[KNL] Number of times to repeat the stall sequence, so that rcutorture.stall_cpu_repeat=3 will result in four stall sequences.

refscale.lookup_instances=

[KNL] Number of data elements to use for the forms of SLAB_TYPESAFE_BY_RCU testing. A negative number is negated and multiplied by nr_cpu_ids, while zero specifies nr_cpu_ids.

smp.panic_on_ipistall=

[KNL] If a csd_lock_timeout extends for more than the specified number of milliseconds, panic the system. By default, let CSD-lock acquisition take as long as they take. Specifying 300000 for this value provides a 5-minute timeout.

spectre_bhi=

[X86] Control mitigation of Branch History Injection (BHI).

tsa=

[X86] Control mitigation for Transient Scheduler Attacks on AMD CPUs. Search the following in your favourite search engine for more details:

clocksource.max_cswd_read_retries=

[KNL] Number of clocksource_watchdog() retries due to external delays before the clock will be marked unstable. Defaults to two retries, that is, three attempts to read the clock under test.

disable_cpu_apicid=

[X86,APIC,SMP] Format: <int> The number of initial APIC ID for the corresponding CPU to be disabled at boot, mostly used for the kdump 2nd kernel to disable BSP to wake up multiple CPUs without causing system reset or hang due to sending INIT from AP to BSP.

nohz_full=

[KNL] Disable the tick when a single task runs as well as disabling other kernel noises like having RCU callbacks offloaded. This is equivalent to the nohz_full parameter. A residual 1Hz tick is offloaded to workqueues, which you need to affine to housekeeping through the global sysfs interface.

mce=

[X86-64] See Documentation /arch/x86/x86_64/boot-options.rst.

mem_encrypt=

[X86-64] See Documentation /virt/kvm/x86/amd-memory-encryption.rst for details on when memory encryption can be activated.

mitigations=

[ALL] Selecting mitigations=off is equivalent to also turning off the following:

pci=config_acs=

[PCI] Format: <ACS flags>@<pci_dev>[; …​]

pirq=

[SMP,APIC] See Documentation /arch/x86/i386/IO-APIC.rst.

prot_virt=

[S390] Enable hosting protected virtual machines isolated from the hypervisor (if hardware supports that). If enabled, the default kernel base address might be overridden even when Kernel Address Space Layout Randomization is disabled. Format: <bool>

sev=

[X86-64] See Documentation /arch/x86/x86_64/boot-options.rst.

spectre_v2_user=

[X86] Control mitigation of Spectre variant 2 for user space. Selecting on will also enable the mitigation against user space to user space task attacks. Selecting specific mitigation does not force enable user mitigations. Selecting off will disable both the kernel and the user space protections.

rcutorture.stall_cpu_irqsoff=

[KNL] Disable interrupts while stalling if set, but only on the first stall in the set.

timer_migration

When set to a non-zero value, attempt to migrate timers away from idle CPUs to allow them to remain in low power states longer. Default: 1