[PTLsim-devel] PTLSim on AMD-64

Wed Apr 16 05:36:04 EDT 2008

> The problem was instability, with multicore FullSystem simulations.

There was one major problem with multi-threaded / multi-core simulation:
The atomic RMW instructions (which are used to realise mutexes, locks and
other similar constructs e.g. inside the linux kernel, libc and
pthreads) were not
actually atomic. I've sent a patch for that quite a while ago, which fixed most
of my instabilities in the past.

In addition to that, there are serious stability issues when switching back and
forth between simulation and native execution. Matt has once suggested that
this might be due to races between the hypervisor taking over and PTLsim
releasing the once simulated core "into the wild".

> Anyway, this problem will probably last while PTLSim is bounded to Xen. We
> hope that will change soon.

I'm again working on PTLsim now, and fixing this issue is on my list of things.
At some point Matt had a developer position available for someone who would
port PTLsim to use QEMU (or KVM?) instead of Xen as a platform. I don't know
how much progress has been made so far, perhaps his offer is still valid.

In addition: Please do not only hope. This stuff is open-source. I guess every
help in fixing is appreciated.

> BTW, the multicore simulation is not yet cycle accurate! And if you're doing
> single core full-system simulations you're probably OK, with both stability
> and accuracy.

Neither is the single-core. I have a patch upcoming that adds true
multiple cores
and a _minimal_ coherency protocol. Stay tuned!

Cheers,
  Stephan