How to enable SYSENTER/SYSEXIT in glibc-2.7?

[Guofu Xiang]

I have known that from glibc-2.3.1-25, SYSENTER is supported as the default.
Now, My OS is Fedora 8(kernel version:
linux-2.6.23), and glibc-2.7-2 is the default. When I compile C program,
this is the debug result by gdb:

     (gdb) disas __kernel_vsyscall
     Dump of assembler code for function __kernel_vsyscall:
     0xb7f7a400 <__kernel_vsyscall+0>:       int    $0x80
     0xb7f7a402 <__kernel_vsyscall+2>:       ret
     End of assembler dump.

     The system call is implemented by int $80h, rather than SYSENTER. What
is the reason? How to set SYSENTER as the default implementation of system
call? Thank you for your response!