The Linux kernel is preparing to remove its last remaining MMX-specific optimization: code used for XOR operations in software RAID. The patch, submitted by Google’s Eric Biggers, is in Andrew Morton’s MM tree and could be included in Linux 7.5.
The optimization applies to 32-bit processors without SSE support, including the Intel Pentium MMX and Pentium II. If the code is removed, those CPUs running Linux software RAID will use the kernel’s existing scalar C implementation instead.
The patch description says the MMX path is limited to roughly 25-year-old processors and systems with a RAID array. It also notes that the scalar alternative is already well tested.
This RAID code is the kernel’s last MMX-optimized implementation. MMX optimizations for memory-management functions such as memcpy(), copy_page() and clear_page() were removed earlier, while the remaining RAID code had become an exception. The patch also updates CPU feature checks in the RAID6 code.
Sources: Phoronix.