Debug: Add mcontext/scontext registers for breakpoint qualification

[ernie-sifive]

Related issue:

Type of change: feature request

Impact: API addition (no impact on existing code)

Development Phase: implementation

Release Notes

The Debug Spec defines mcontext and scontext as CSRs that M-mode and S-mode OSs can write to when switching tasks. The OS writes a process ID or similar identifier to uniquely identify the incoming task or process. A debugger may choose to qualify breakpoints by requiring that either mcontext or scontext (or both) match values programmed into the textra register of each breakpoint. This allows triggering on a virtual address that might be used by more than one process.

mcontext and scontext have a secondary use as additional comparison registers that an M-mode or S-mode application can write to enable and disable breakpoints. So even if a core doesn't have S-mode, scontext can still be a useful feature.

This feature is enabled with rocketCoreParams.mcontextWidth > 0 and/or rocketCoreParams.scontextWidth > 0. The default is disabled.