[Qemu-devel] Weird behavior while using the instruction counter

[Luis Pureza]

Hi,

I'm using the instruction counter to execute N instructions at a time.
With very small values of N (say, N < 10), I observed the following
behavior:

1. A new TB is generated and execution starts there;
2. The instruction counter timer expires and cpu_exec_nocache() is called;
3. cpu_exec_nocache() generates a new TB for the same PC and starts to
execute it;
4. Some instruction inside the TB turns out to be an I/O instruction.
Thus, cpu_io_recompile() gets called
5; cpu_io_recompile() regenerates the TB and longjmps back to the
beginning of cpu_exec()
6. on cpu_exec(), tb_find_fast() returns the first TB, instead of the
one generated by cpu_io_recompile()
7. Endless loop!

Actually, for some reason beyond my comprehension, the loop is not
really infinite: after a few seconds it actually executes the block
and moves on. However, as you can imagine, this is too slow.

I think I fixed the problem by appending CF_LAST_IO to the cflags of
the TB generated by cpu_exec_nocache(). This way, cpu_io_recompile()
won't be called for this TB. Certainly not the best solution, though.

I was wondering if my thinking makes sense or am I just completely wrong :-)

Cheers,

Luis Pureza