What is the meaning of the verse?
Rashi: Your salvation comes to me like Your word, that You promised to me.
Radak: Your salvation and Chesed come to me like You promised to me.
Malbim: I explained (Mizmor 89 1 ) that Kisei David is based on Chesed and Emunah. Initially Hashem established for him a Bayis Ne'eman based on Chesed. After He promised him and made a Bris with him, now it is Emes and it is as if Hashem is forced to fulfill His word. If a promise was based on deeds, if he sins, the good will depart from him. If it was based on Chesed, and not merit, if he sins, Hashem punishes immediately to eradicate the sin, and the Chesed will not depart. Here David requests, since the Bris was amidst Chesed, Your salvation will come to guard the promise, even if he sins.
Malbim (89:29): Beis David was initially due to Emunah (David's good deeds). However, based on deeds, He was not obliged to build for him a Bayis Ne'eman, that kingship will be hereditary; this was via Chesed. I.e. He will prepare that his sons never falter from good deeds, so the Chesed will not depart from them. He will make them return to the good. (Here, Malbim says that it was initially based on Chesed! Perhaps Beis David was initially via Emunah, but Kisei David was initially via Chesed. This requires investigation. - PF)
