IDENTITY_FRAME_BIT
mfn &= ~(FOREIGN_FRAME_BIT | IDENTITY_FRAME_BIT);
#define IDENTITY_FRAME(m) ((m) | IDENTITY_FRAME_BIT)
mfn &= ~(FOREIGN_FRAME_BIT | IDENTITY_FRAME_BIT);
if (mfn & IDENTITY_FRAME_BIT)