John Gill Commentary 1 Samuel 29:6

"Then Achish called David, and said unto him, As Jehovah liveth, thou hast been upright, and thy going out and thy coming in with me in the host is good in my sight; for I have not found evil in thee since the day of thy coming unto me unto this day: nevertheless the lords favor thee not." — 1 Samuel 29:6 (ASV)

Then Achish called David
Being so near him, that he could call to him himself, or he sent some person to him, to require his presence with him:

and said to him, surely, [as] the Lord liveth ;
or "Jehovah liveth"; an oath by the true God, of whom Achish might have some knowledge, as he also had of angels, from his conversation with David; though the Heathens had a notion of a supreme Being, and yet worshipped other gods, and whom they called Jove, from this name of Jehovah. Kimchi observes, that all confess a first cause; and therefore when he swore to David, he swore by him in whom David believed, perhaps out of complaisance to him, or that David might pay the greater regard to his oath:

you have been upright ;
sincere, honest, faithful, and just in all his deportment; yet not so sincere as he thought him to be, witness the road he pretended he had taken against the south of Judah, (1 Samuel 27:1–12) ;

and your going out and your coming in with me in the host [is] good in
sight ;
his behaviour in the army, attending him as the keeper of his head, or captain of his bodyguard, was exceeding agreeable to him, and he could wish to have him continued:

for I have not found evil in you, since the day of your coming to me
unto this day ;
whatever he had done before to the Philistines, having greatly afflicted and distressed them in his wars with them, of which this seems to be an exception:

nevertheless, the lords favour you not ;
or you are not acceptable to them, yes, very offensive and disagreeable.