ugdb && breakpoints

[Tom Tromey]

Oleg> 	Now, to continue the tracee, gdb does not restore the
Oleg> 	original instruction. Instead, it
Oleg> 		- writes this insn into _start code
Oleg> 		- changes regs->ip to point to this insn
Oleg> 		- does single-step to execute this insn
Oleg> 		- changes regs->ip again

This is what is done for non-stop.
I believe it is called "displaced stepping" in gdb.

I think eventually we would like it if uprobes did this work, instead of
gdb doing it.  Presumably that would yield better performance.  E.g., if
we have a thread-specific breakpoint, then other threads hitting that
breakpoint could simply do the displaced stepping via uprobes, and not
report a breakpoint hit to gdb at all.

For all-stop, breakpoints are handled differently, though I don't
remember how offhand.

Tom