DeMarcus Cousins -- NBA Peace Summit ... Coach Was A Snake, But Now We're Cool (PHOTO)

DeMarcus Cousins and Kings coach George Karl HATE each other on a Drake-Meek Mill level ... well, at least they did until today ... when both men posed looking like longtime homies.

The beef ran deep. Earlier in the summer ... after word leaked Karl wanted to trade his star center ... Boogie (very creatively), using 3 emojis, called his coach a snake in the grass on Twitter.

So, game over. Relationship over, right? Nope. Shout out to our friends at BSO for finding this gem -- the 2 thought-to-be mortal enemies ... smiling with arms wrapped around each other for an Instagram pic.

That sound you hear is a DEEP sigh of relief from Kings fans and front office staffers.