Linux 6.1

I'm a little behind on stuff since I'm waiting for parts to get my T2 booting again (doing everything on my Mac laptop and my long-suffering Quad G5), but kernel version 6.1 came out, and there's some really good stuff on Power to mention.

But first the marquee general improvements: first, general support for Rust in kernel, which is now fairly mature on Power ISA (every Firefox build I make has it) and has obvious security benefits — assuming you're on a platform it supports, that is. The other change I think is a big one, possibly even bigger than Rust support, is the enhanced multi-generational LRU (Least Recently Used) memory page evictor: it's not on by default, but it ships as a configurable option, and some of the reports show some impressive performance wins. Finally, the new implementation of in-kernel maple trees means better cache hit rates and less lock contention for those kernel structures reimplemented with them (if you're 64-bit and have an MMU, which naturally we do), and I know people will appreciate the updates to AMD GPU support.

However, the Power-specific improvements are particularly interesting. If you're using the POWER9-and-up radix MMU (not available on the POWER8, nor if you need to use KVM-PR thanks Russell Currey for the correction: HPT already has this support), there's now the option of execute-only mapping (as opposed to read-execute which is supported with hashed page tables). Another important Power improvement is full support for 64-bit Power ISA under both hashed and radix MMUs with KFENCE, a "low-overhead sampling-based memory safety error detector of heap use-after-free, invalid-free, and out-of-bounds access errors." Interestingly, 32-bit PowerPC was supported first!

To me, though, I'm most impressed with the exceptionally hard but worthy work done to rework system calls to use the new shared syscall wrapper implemented for s390, arm and x86 and obsolete the old legacy layer. This causes syscall handlers to take their parameters off the stack rather than relying on the state of the argument registers and r0, which is an obvious benefit if the registers are already on stack (such as for exception handling) because an additional stack frame wouldn't be needed, and further offers the opportunity to zero or sanitize them to prevent them from being used as a means to influence speculative execution (where expedient, and likely coming in 6.2). This has at most a minor performance boost, but it seems to be a definite security and maintainability gain, and best of all the new wrappers work on all PowerPC and Power ISA CPUs except the IBM Cell.

Expect to see it soon in Fedora and other leading-edge builds, and trickling down to other distros near you (full change list).


  1. Hey, thanks for the writeup! Just want to clarify that execute-only mappings were already supported with the Hash MMU, it's just implemented using protection keys instead of it being a "native" MMU feature like it is for Radix. A PROT_EXEC-only mapping in Linux 6.1 should behave the same on both Hash and Radix as far as users can tell.

    1. Cool! Thanks for mentioning it. I've updated the article.


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