xenbus_exists
fb_exists = xenbus_exists(xsname, XBP_FB);
if (!(vdp->xdf_feature_barrier = xenbus_exists(oename, XBP_FB)))
vdp->xdf_media_req_supported = xenbus_exists(oename, XBP_MEDIA_REQ_SUP);
if (XD_IS_CD(vdp) && !xenbus_exists(oename, XBP_MEDIA_REQ_SUP)) {
extern boolean_t xenbus_exists(const char *dir, const char *node);