All operating systems with separate user and kernel modes have a privilege-level round-trip on every syscall (typically `sysenter`/`sysexit`, on older systems t

All operating systems with separate user and kernel modes have a privilege-level round-trip on every syscall (typically `sysenter`/`sysexit`, on older systems the classic `int $0x80`/`iret`). This is just a controlled jump that changes the privilege level, and is what is bypassed by vsyscall. Non-shared-cr3 Macs (and IIRC some versions of PaX) also change `%cr3`, which means user-space and kernel-space have completely different address spaces (rather than a shared kernel-space and per-process user-space). This is much more expensive.