Normal-mode relaxation of entangled cis-polyisoprene (PI) confined in porous glass was studied using broadband dielectric spectroscopy. In our designed model systems, geometric confinement effects are much weaker than surface adsorption effects, allowing relaxation dynamics of polymer chains at the surface to be determined from the dielectric response. PI confined in glass pores exhibits two relaxation processes: one fast mode and one slow mode. The fast dynamics are almost independent of polymer molecular weight and appear to be controlled by dynamic adsorption/desorption of chain segments at the surface. The slow dynamics correspond to the global chain relaxation, which is broadened and shifted to lower frequencies compared with the bulk process. The characteristic relaxation time of the slow mode tau(s) shows much stronger molecular weight M dependence (e.g., tau(s) similar to M-5.7 +/- (0.15) at 30degreesC) than the bulk relaxation time tau(b) (tau(b) similar to M-3.5). Another noteworthy finding is that the activation energy of the slow mode increases with M. These results indicate an increase of surface confinement effects as M increases. It is also found that surface confinement effects decrease with increasing temperature and can be largely eliminated by coating the pore walls with a monolayer of oligomers to prevent surface adsorption. The structure of PI at surfaces and molecular mechanisms available for relaxation of adsorbed chains are discussed.