SW4STM32 and SW4Linux fully supports the STM32MP1 asymmetric multicore Cortex/A7+M4 MPUs

   With System Workbench for Linux, Embedded Linux on the STM32MP1 family of MPUs from ST was never as simple to build and maintain, even for newcomers in the Linux world. And, if you install System Workbench for Linux in System Workbench for STM32 you can seamlessly develop and debug asymmetric applications running partly on Linux, partly on the Cortex-M4.
You can get more information from the ac6-tools website and download (registration required) various documents highlighting:

System Workbench for STM32

Debugging STM32L476 with System Workbench

Sorry, I’m not familiar with the debugger internals.

However I have had System Workbench suddenly lose the ability to connect/download to a target. When that happens I create a new empty project and get that working. Once it does, if the problem was not hardware related I copy my code to the new project as it is faster that figuring out what is wrong with the configuration files in the old project.