On STM32H747IGT6, HSEM channel 0 is reserved?
Application note AN5361 "Setting up with ST-LINK GDB server" says "Set the Cortex®-M7 core running so that the HSEM semaphore is released and Cortex®-M4 wakes up from Stop mode" and this post implies that it's "HSEM 0" which I'm assuming is HSEM chan...