[amluto]

Huh? On x86, a syscall/sysret pair takes a couple hundred cycles, doesn't serialize, and doesn't zap caches.

It's the other ugly state changes that hurt a lot. Switching address spaces burns a few hundred cycles and zaps the TLB (fix coming in Linux 3.7, maybe). Interrupts are a few thousand cycles.

[cthalupa]

It is something to take into account if you're utilizing PV based virtualization in Xen, however - system calls will require a context switch.