FOREIGN_FRAME_BIT
mfn &= ~(FOREIGN_FRAME_BIT | IDENTITY_FRAME_BIT);
#define FOREIGN_FRAME(m) ((m) | FOREIGN_FRAME_BIT)
mfn &= ~(FOREIGN_FRAME_BIT | IDENTITY_FRAME_BIT);
if (mfn != INVALID_P2M_ENTRY && (mfn & FOREIGN_FRAME_BIT))