Chapter 5. Important changes to external kernel parameters

With this parameter you can allow systems with mismatched 32-bit support at the EL0 level to run 32-bit applications. The set of CPUs supporting 32-bit EL0 is indicated by the /sys/devices/system/cpu/aarch32_el0 file. Also, you can restrict hot-unplug operations.

For more information, see Documentation/arm64/asymmetric-32bit.rst.

With this parameter in the <bool> format, you can allow override of default traffic class configuration for the device.

The default value is set to false (0).

With this parameter you can control whether or not a KVM proactively splits all huge pages during dirty logging. Eager page splitting reduces interruptions to vCPU execution by eliminating the write-protection faults and Memory Management Unit (MMU) lock contention that is otherwise required to split huge pages lazily.

VM workloads that rarely perform writes or that write only to a small region of VM memory can benefit from disabling eager page splitting to allow huge pages to still be used for reads.

The behavior of eager page splitting depends on whether the KVM_DIRTY_LOG_INITIALLY_SET option is enabled or disabled.

With this parameter you can control the time period at which KVM zaps 4 KiB pages back to huge pages.

With this parameter you can control mitigation for L1D-based snooping vulnerability.

Certain CPUs are vulnerable to an exploit against CPU internal buffers which can, under certain conditions, forward information to a disclosure gadget. In vulnerable processors, the speculatively forwarded data can be used in a cache side channel attack, to access data to which the attacker does not have direct access.

The available option is on, which means enable the interface for the mitigation.

With this parameter you can control mitigation for the Processor Memory-mapped I/O (MMIO) Stale Data vulnerabilities.

Processor MMIO Stale Data is a class of vulnerabilities that can expose data after an MMIO operation. Exposed data could originate or end in the same CPU buffers as affected by metadata server (MDS) and Transactional Asynchronous Abort (TAA). Therefore, similar to MDS and TAA, the mitigation is to clear the affected CPU buffers.

The available options are:

With this parameter you can set the number of callback queues to use for the RCU Tasks family of RCU flavors. You can adjust the number of callback queues automatically and dynamically with the default value of -1.

This parameter is intended for use in testing.

With this parameter you can control mitigation of Arbitrary Speculative Code Execution with Return Instructions (RETBleed) vulnerability. The available options are:

Format: { s3_bios, s3_mode, s3_beep, s4_hwsig, s4_nohwsig, old_ordering, nonvs, sci_force_enable, nobl }

With this parameter you can allocate physical memory region from top as follows:

When you pass crashkernel=X,high, the kernel can allocate a physical memory region above 4 GB. This causes the second kernel crash on systems that require some amount of low memory (for example, swiotlb requires at least 64M+32K low memory) and enough extra low memory to make sure DMA buffers for 32-bit devices are not exhausted. Kernel tries to allocate at least 256 M below 4 GB automatically. With this parameter you can specify the low range under 4 GB for the second kernel instead.

With this parameter you can control how many 4 KiB pages are periodically zapped back to huge pages:

With this parameter you can select one of KVM modes of operation:

With this parameter you can control optional mitigations for CPU vulnerabilities. This is a set of curated, arch-independent options, each of which is an aggregation of existing arch-specific options:

The optional argument is a CPU list.

In kernels built with CONFIG_RCU_NOCB_CPU=y, you can enable the no-callback CPU mode, which prevents such CPUs callbacks from being invoked in softirq context. Invocation of such CPUs' RCU callbacks will instead be offloaded to rcuox/N kthreads created for that purpose, where x is p for RCU-preempt, s for RCU-sched, and g for the kthreads that mediate grace periods; and N is the CPU number. This reduces OS jitter on the offloaded CPUs, which can be useful for HPC and real-time workloads. It can also improve energy efficiency for asymmetric multiprocessors.

With this parameter you can set the SCHED_FIFO priority of the RCU per-CPU kthreads (rcuc/N). This value is also used for the priority of the RCU boost threads (rcub/N) and for the RCU grace-period kthreads (rcu_bh, rcu_preempt, and rcu_sched).

With this parameter you can specify the number of kthreads to be used for RCU grace-period forward-progress testing for the types of RCU supporting this notion.

The default is set to 1 kthread. Values less than zero or greater than the number of CPUs cause the number of CPUs to be used.

With this parameter you can control mitigation of Spectre variant 2 (indirect branch speculation) vulnerability. The default operation protects the kernel from user space attacks.

With this parameter you can control user space access for reading perf event counters.

With this parameter you can choose order-0 allocation. By default, the allocator for page fragments tries to use high order pages, that is order-3 on X86 systems. While the default behavior returns good results, in certain situations a contention in page allocations and freeing occurs. This was especially true on older kernels (version 5.14 and higher) when high-order pages were not stored on per-CPU lists. This parameter exists now mostly of historical importance.

The default value is 0.

By specifying the value for this parameter you can determine the number of times that the page lock can be stolen from under a waiter. After the lock is stolen the number of times specified in this file, the fair lock handoff semantics will apply, and the waiter will only be awakened if the lock can be taken.

The default value is 5.