Linux Kernel 6.14 rc3 Released – What’s New!

Linus Torvalds has released Linux Kernel 6.14-rc3, the latest release candidate for the upcoming Linux 6.14 stable version.

Paolo Bonzini, the maintainer of the Kernel-based Virtual Machine (KVM), has also submitted a series of fixes for the Linux Kernel 6.14-rc3, which have now been merged by Linus Torvalds.

These updates address critical issues across multiple architectures, including ARM and x86, while also introducing improvements to KVM’s functionality and stability.

This release also includes significant updates, including the introduction of the Faux Bus, a new feature aimed at simplifying device driver development, alongside a range of critical fixes.

The Faux Bus is a lightweight framework designed for simple devices and drivers that do not require the complexity of a full platform device driver. This feature offers a minimalistic API with just two functions one for creating devices and another for removing them.

It is particularly useful for small or “fake” devices, streamlining their lifecycle management while maintaining simplicity. The Faux Bus also supports both C and Rust language bindings, enabling broader adoption by developers.

Key Highlights of the KVM Updates

ARM Architecture:

• A comprehensive overhaul of vector register handling has been implemented to simplify floating-point (FP), SIMD, and Scalable Vector Extension (SVE) management. This update resolves numerous bugs affecting real-world deployments.
• Fixes were applied to address a race condition between virtual CPU (vCPU) and virtual Generic Interrupt Controller (vGIC) initialization.
• Adjustments were made to prevent the use of kernel virtual addresses (VAs) at EL2 when emulating timers in non-VHE (Virtualization Host Extensions) mode.
• Improvements were introduced for Protected KVM (pKVM), including enhanced error handling and memory alignment fixes.

x86 Architecture:

• Support for AMD Secure Nested Paging (SNP) was fixed to ensure proper initialization of the Platform Security Processor (PSP) module when the KVM module is built-in.
• Hyper-V-related improvements include rejecting unsupported SEND_IPI hypercalls when local Advanced Programmable Interrupt Controllers (APICs) are not emulated by KVM.
• Bug fixes include addressing stale DR6 register values during guest transitions and ensuring proper tagging of nested page tables for Level 2 guests.

General Enhancements:

• Multiple architectural cleanups were performed, including better error handling and simplifications in code paths.
• Self-tests for KVM were expanded to cover additional scenarios, such as Hyper-V CPUID features and nested virtualization edge cases.

The patch series includes changes across 30 files, with 420 lines added and 427 lines removed. The updates reflect a balance between bug fixes, performance optimizations, and code simplifications.

Developer Insights

Bonzini highlighted that while this pull request is relatively large due to significant ARM updates, it removes more code than it adds a testament to the ongoing effort to streamline KVM’s codebase. Mark Rutland’s contributions to ARM vector handling were particularly noted for their complexity and impact on stability.

As Linux Kernel 6.14 progresses through its release candidate phase, further testing is expected to refine these updates before the final release. Developers are encouraged to test these changes in diverse environments and report any issues.

The KVM updates continue to underscore the importance of collaboration within the Linux community in maintaining robust virtualization support across architectures.

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