Xtensa is a licensable IP architecture from Tensilica / Cadence Design Systems. While configurable, the Xtensa CPU architecture is based on a 32-bit RISC core.
Xtensa support has long been part of the mainline Linux kernel but now with the Linux 3.14 kernel there's SMP support for leveraging multiple Xtensa processor cores.
Other Xtensa updates for the Linux 3.14 kernel can be found by this Git pull.
Other new features that can be found so far in the Linux 3.14 kernel that's under early stages of development are covered in this early feature overview from Friday.