Differences between powerpc64* SLB/PTE architectures:

* PPC970 (PowerMac G5) are based on PowerPC ISA, *not* PowerISA 2.03, even
  though it's based on the POWER4.
  * PPC970 does not allow for mixed page sizes in a single SLB
  * `tlbie` format takes 2 arguments:
    * tlbie %rB, L
      * %rB is the cropped AVPN (top 16 bits must be 0).
      * L bit is a constant defining large page or small page.
* PowerISA 2.03 allows for mixed page sizes, but this looks convoluted.
  * `tlbie` format is similar to PPC970's:
    * tlbie %rB, L
      * L is a constant as for PPC970, if 0, then the `AP` field determines if
      	the page is 4k or 64k.  If 1, the page size (real and virtual) is
      	encoded in the `LP` field of %rB
      * %rB encodes the VPN and page size.
* PowerISA 2.06 (POWER7) and later all allow for mixed page sizes in a single
  SLB, thus allowing for transparent superpages.  This may also apply to ISA
  2.04 (Power5) but has not been confirmed.
  * `tlbie` takes 2 arguments (more for 3.1)
    * tlbie %rB, %rS
      * %rB is an encoding of the AVPN, page size, SLB size
        * Bit 63 (LSB) denotes if large page encoding or not ('L')
        * If L=0, `AP` encoding denotes the PTE(L||LP) or SLB(L||LP), used in
          FreeBSD, this is 0b100, to denote 16MB superpage on 4k base page size
        * If L=1, `LP` encodes real and actual page size; this is unused on
          FreeBSD because we only use 4k pages
      * %rS encodes (PID||LPID), each (PID and LPID) are 32 bits