KPTI is based on KAISER. Without KPTI enabled, whenever executing user-space code (applications), Linux would also keep its entire kernel memory mapped in page tables, although protected from access. The advantage is that when the application makes a system call into the kernel or an interrupt is received, kernel page tables are always present, so most context switching-related overheads (TLB flush, page-table swapping, etc) can be avoided.
