(abridged) Two XMM-Newton observations of the black-hole binary GRS1915+105 were triggered in 2004, during a long "plateau" state of the source. (...) While the light curves show just small amplitude variations (a few percent) at timescales longer than a few seconds, a QPO is seen at about 0.6 Hz (...). The pn spectrum is well fitted without invoking thermal disk emission, on the base of four main components: a primary one (...), absorbed by cold matter with abundances different than those of standard ISM; reprocessing from an ionized disk; emission and absorption lines; and a soft X-ray excess around 1 keV. However, the latter is not confirmed by the RGS spectra, whose difference from the EPIC-pn ones actually lacks of a fully satisfactory explanation. If real, the soft X-ray excess may be due to reflection from an optically thin, photoionized disk wind; in this case it may yield a way to disentangle intrinsic from interstellar absorption.