sysenter

The low 16 bits of IA32_SYSENTER_CS select the kernel's code segment, with its stack segment in the next descriptor; IA32_SYSENTER_EIP and IA32_SYSENTER_ESP hold the entry point and stack pointer, loaded in full under a 64-bit kernel and as their low 32 bits otherwise. It also clears IF and VM. It saves no return address: the caller and the kernel agree on where to resume, and sysexit returns there.

It is the 32-bit system call path. A 64-bit Linux program enters the kernel with syscall.