Contains information relevant to the kernel:

Allows us to specify an override for the CPU ID register so that the kernel can pretend it is a "known" CPU type. This is done because the kernel "knows" only about certain types of PPC CPUs; different variants require specialized support. When a new variant is manufactured, the kernel will not recognize it. By stuffing the pretend_cpu field with a CPU ID from a known CPU, the kernel will pretend that it's running on the known variant.
Template of what bits to have on in the MSR when creating a thread. Since the MSR changes among the variants in the PPC family, this allows you to specify some additional bits that the kernel doesn't necessarily know about.
Indicates what family the PPC CPU belongs to.
Identifies what address space bits are active.
Lets callouts know whether to turn off data translation to get at their hardware.