[Qemu-devel] Holding the BQL for emulate_ppc_hypercall

["Alex Bennée" <alex.bennee@linaro.org>]

Hi,

In the MTTCG patch set one of the big patches is to remove the
requirement to hold the BQL while running code:

  tcg: drop global lock during TCG code execution

And this broke the PPC code because emulate_ppc_hypercall can cause
changes to the global state. This function just calls spapr_hypercall()
and puts the results into the TCG register file. Normally
spapr_hypercall() is called under the BQL in KVM as
kvm_arch_handle_exit() does things with the BQL held.

I blithely wrapped the called in a lock/unlock pair only to find the
ppc64 check builds failed as the hypercall was made during the
cc->do_interrupt() code which also holds the BQL.

I'm a little confused by the nature of PPC hypercalls in TCG? Are they
not all detectable at code generation time? What is the case that causes
an exception to occur rather than the helper function doing the
hypercall?

I guess it comes down to can I avoid doing:

  /* If we come via cc->do_interrupt BQL may already be held */
  if (!qemu_mutex_iothread_locked()) {
      g_mutex_lock_iothread();
      env->gpr[3] = spapr_hypercall(cpu, env->gpr[3], &env->gpr[4]);
      g_muetx_unlock_iothread();
  } else {
      env->gpr[3] = spapr_hypercall(cpu, env->gpr[3], &env->gpr[4]);
  }

Any thoughts?

--
Alex Bennée