pagezero() was a plain store loop (bzero), which under the kernel build flags (-mno-vsx -msoft-float) compiles to byte stores. dcbz establishes a zeroed cache line directly in the cache without a read-for-ownership fetch from memory, roughly halving the memory transactions of page zeroing. Measured on POWER9 (Raptor Blackbird, DD2.3, bare metal), zeroing a cold 256 MB buffer with 128-byte scalar loops: byte stores (current libkern memset) 8.6 GB/s doubleword (std) stores 26.8 GB/s dcbz 34.6 GB/s dcbz raises an alignment interrupt on caching-inhibited mappings, and the kernel does not emulate it, so mmu_radix_zero_page() falls back to bzero() for any page whose memattr is not the write-back default. The internal pagezero() callers only touch freshly allocated page-table pages, which are always write-back. Note: dcbz helps only zeroing, where there is no source to read. For page copying it is a pessimization (it adds a redundant zeroing pass on top of the mandatory source read), so mmu_radix_copy_page() is left as a plain bcopy(). Differential Revision: https://reviews.freebsd.org/D59507 Reviewed by: jhibbits