DAWR YOLO even with DD2.3

Way back in Linux 5.2 was a "YOLO" mode for the DAWR register required for debugging with hardware watchpoints. This register functions properly on POWER8 but has an erratum on pre-DD2.3 POWER9 steppings (what Raptor sells as "v1") where the CPU will checkstop — invariably bringing the operating system to a screeching halt — if a watchpoint is set on cache-inhibited memory like device I/O. This is rare but catastrophic enough that the option to enable DAWR anyway is hidden behind a debugfs switch.

Now that I'm stressing out gdb a lot more working on the Firefox JIT, it turns out that even if you do upgrade your CPUs to DD2.3 (as I did for my dual-8 Talos II system, or what Raptor sells as "v2"), you don't automatically get access to the DAWR even on a fixed POWER9 (Fedora 34). Although you'll no longer be YOLOing it on such a system, still remember to echo Y > /sys/kernel/debug/powerpc/dawr_enable_dangerous as root and restart your debugger to pick up hardware watchpoint support.

Incidentally, I'm about two-thirds of the way through the wasm test cases. The MVP is little-endian POWER9 Baseline Interpreter and Wasm support, so we're getting closer and closer. You can help.