Linux DMA Mapping Updates Help Recover Losses Caused By Spectre V2 Retpolines

Yet another pull request sent in early ahead of the holidays for the Linux 4.21 kernel merge window are the DMA-Mapping updates managed by Christoph Hellwig. Normally the DMA-Mapping changes aren't really worth noting on Phoronix, but this time around it brings some improvements to help offset the overhead incurred by Retpolines for Spectre V2 mitigation...