One thing I would find useful is the ability to access register values during an execution-time callback. I think the easiest way to do that generically would be to expose them via the gdb functionality (like Pavel's earlier patchset did [1]), though that (currently) limits you to the general-purpose registers. Ideally it would be nice be able to access other registers (i.e. floating-point, or maybe even system registers), though those are more difficult to do generically.
Perhaps if we added some sort of architectural-support checking for individual plugins like I mentioned in another response to this patchset, we could allow some limited architecture-specific functionality in this vein? I confess I haven't thought through all the ramifications of that yet, though. -Aaron [1] - See qemulib_read_register() at https://patchwork.ozlabs.org/patch/925393/