What is the exclusionary rule in the context of search warrants?

We explained the rationale behind this rule in People v. Keener, supra, 55 Cal.2d at pages 722-723, 12 Cal.Rptr. 859, 361 P.2d 587: "If a search is made pursuant to a warrant valid on its face and the only objection is that it was based on information given to a police officer by an unnamed informant, there is substantial protection against unlawful search and the necessity of applying the exclusionary rule in order to remove the incentive to engage in unlawful searches is not present. The warrant, of course, is issued by a magistrate, not by a police officer, and will be issued only when the magistrate is satisfied by the supporting affidavit that there is probable cause. He may, if he sees fit, require disclosure of the identity of the informant before issuing the warrant or require that the informant be brought to him. The requirement that an affidavit be presented to the magistrate and his control over the issuance of the warrant diminish the danger of illegal action, and it does not appear that there has been frequent abuse of the search warrant procedure.... It follows from what we have said that where a search is made pursuant to a warrant valid on its face, the prosecution is not required to reveal the identity of the informer in order to establish the legality of the search and the admissibility of the evidence obtained as a result of it." (Italics added.)