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Attosecond XUV-pump XUV-probe experiments demand high brightness attosecond light source benefitting from state-of-the-art ultrashort ultraintense laser technology. Current most promising route towards high energy attosecond light source is through relativistic high-order harmonic generation from plasma surfaces. In this paper, we investigate waveform-dependent relativistic high-order harmonic generation from plasma surfaces, and use spectral interferometry to understand its generation process. The unique interpretation has allowed access to unrevealed temporal structure of the generated few-pulse attotrain with evidence supporting a well-isolated attosecond pulse. It also provides a way to measure field-driven plasma surface motion in its generation process.