Ipc_attach() hangs on the DSP of DM8168

I am trying an application on Dm8168 EVM board which uses Syslink (version 2_21_01_05) .
I have created my new platform for DM8168 with xdctools.
But when I run the application, it hangs in the call to Ipc_control(STARTCALLBACK)
I checked the examples,  And I found if I add "Cache = xdc.useModule('ti.sysbios.family.c64p.Cache');
Cache.MAR128_159 = 0x00008000;"  to cfg file , it works.
Why should I do so?
I haven't found any notes about it.