systems calls on linux that deal with per-process state try very hard to not invalidate userland memory mappings. The kernels real range and userland's virtual

systems calls on linux that deal with per-process state try very hard to not invalidate userland memory mappings. The kernels real range and userland's virtual range won't overlap so for a lot of functions kernel memory just has to mapped/unmapped on call, not invalidate _all_ userland bindings. Well okay they will overlap. So yeah your mappings may get invalidated but for synchronized higher performance systems calls they _shouldnt_. This lets them be in the 10's to 100's of nano-seconds. Normally the most _expensive_ part of a linux syscall is the TLB misses after one. --- Your model of memory transfer size assumes data is being copied. The Linux kernel has a lot of features to let userland, devices, and itself all share the same memory copy free.