sysemu SMP scheduling

[Brian Cain]

For some hexagon use cases, we would prefer to have finer grained scheduling 
among multiple guest cores/threads.  We haven't been able to determine exactly 
what kind of scheduling algorithm is operating in the baseline case.  If the 
current hw thread is ready-to-run and is spinning over a tight loop that never 
hits any exceptions, would it ever yield to another thread after so-many 
iterations/TBs executed?  Or perhaps since we're executing translated blocks 
there's just no yield opportunity available?

We came up with a design for this finer-grained scheduling feature, but are 
re-examining whether or not it should be necessary and if it is necessary, 
whether it should have been designed like so.  We haven't seen a similar 
example in other targets, so we'd love to get feedback on the approach.

In the TranslatorOps .tb_stop() we generate code like so:

        if the current count of ready-to-run threads >= 2:
                tb_count++
                if tb_count > THRESHOLD:
                        gen_exception(EXCP_YIELD);
                        tb_count = 0
                        gen exit_tb

- "current count of ready-to-run threads" is based on the values in the CPU 
state.  When entering a wait/halt mode, we set the appropriate state and call 
cpu_stop_current().
- Is EXCP_YIELD an appropriate mechanism for this feature?  It seems like maybe 
it has no special handling, but any exception can trigger a yield?

-Brian