I know that dS=dQ/T for reversible process,
is dU=TdS-PdV for reversible process only?
This equation has nothing to do with any process. It is a physical property relationship that must be satisfied for any material, describing the relationship between the mutual changes in U, S, and V between and two closely neighboring thermodynamic equilibrium states of the material. This constraint is imposed as a consequence of the 2nd law of thermodynamics.
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