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SYSEXIT

Exits the system call state by loading the RIP from RCX and the RFLAGS from RDX, then returns execution to the CPL specified in the STAR MSR.

The table after the description covers what the source and destinations can be.

DO NOT support LOCK

SYSEXIT is ONLY available in 64-bit mode. It SHALL NOT be used in compatibility mode. The instruction requires that the processor is currently executing at CPL 0.

To avoid an invalid state or general protection fault, the STAR MSR MUST be correctly initialized. If the processor is in 64-bit mode, SYSEXIT will transition the processor to the target CPL (typically CPL 3) and load the segment selectors for CS and SS from the STAR MSR. If RCX or RDX contain invalid addresses or flag combinations, the behavior may result in immediate exceptions upon returning to the target privilege level.