AMD64_PDRSHIFT
pdeindex = (va >> AMD64_PDRSHIFT) & (AMD64_NPDEPG - 1);
#define AMD64_NBPDR (1 << AMD64_PDRSHIFT)
_Static_assert(PDRSHIFT == AMD64_PDRSHIFT, "PDRSHIFT mismatch");
pdeindex = (va - vm->hdr.kernbase) >> AMD64_PDRSHIFT;
va = vm->hdr.kernbase + (pdeindex << AMD64_PDRSHIFT);