Re: [Qemu-devel] [RFC PATCH V6 06/18] tcg: remove tcg_halt_cond global variable.

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/06/2015 16:47, fred.konrad@greensocs.com wrote:
> From: KONRAD Frederic <fred.konrad@greensocs.com>
> 
> This removes tcg_halt_cond global variable.
> We need one QemuCond per virtual cpu for multithread TCG.
> 
> Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
> ---
>  cpus.c | 18 +++++++-----------
>  1 file changed, 7 insertions(+), 11 deletions(-)
> 
> diff --git a/cpus.c b/cpus.c
> index 2d62a35..79383df 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -813,7 +813,6 @@ static unsigned iothread_requesting_mutex;
>  static QemuThread io_thread;
>  
>  static QemuThread *tcg_cpu_thread;
> -static QemuCond *tcg_halt_cond;
>  
>  /* cpu creation */
>  static QemuCond qemu_cpu_cond;
> @@ -919,15 +918,13 @@ static void qemu_wait_io_event_common(CPUState *cpu)
>      cpu->thread_kicked = false;
>  }
>  
> -static void qemu_tcg_wait_io_event(void)
> +static void qemu_tcg_wait_io_event(CPUState *cpu)
>  {
> -    CPUState *cpu;
> -
>      while (all_cpu_threads_idle()) {
>         /* Start accounting real time to the virtual clock if the CPUs
>            are idle.  */
>          qemu_clock_warp(QEMU_CLOCK_VIRTUAL);
> -        qemu_cond_wait(tcg_halt_cond, &qemu_global_mutex);
> +        qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
>      }
>  
>      while (iothread_requesting_mutex) {
> @@ -1047,7 +1044,7 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
>  
>      /* wait for initial kick-off after machine start */
>      while (first_cpu->stopped) {
> -        qemu_cond_wait(tcg_halt_cond, &qemu_global_mutex);
> +        qemu_cond_wait(first_cpu->halt_cond, &qemu_global_mutex);
>  
>          /* process any pending work */
>          CPU_FOREACH(cpu) {
> @@ -1068,7 +1065,7 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
>                  qemu_clock_notify(QEMU_CLOCK_VIRTUAL);
>              }
>          }
> -        qemu_tcg_wait_io_event();
> +        qemu_tcg_wait_io_event(QTAILQ_FIRST(&cpus));

Does this work (for non-multithreaded TCG) if tcg_thread_fn is waiting
on the "wrong" condition variable?  For example if all CPUs are idle and
the second CPU wakes up, qemu_tcg_wait_io_event won't be kicked out of
the wait.

I think you need to have a CPUThread struct like this:

   struct CPUThread {
       QemuThread thread;
       QemuCond halt_cond;
   };

and in CPUState have a CPUThread * field instead of the thread and
halt_cond fields.

Then single-threaded TCG can point all CPUStates to the same instance of
the struct, while multi-threaded TCG can point each CPUState to a
different struct.

Paolo

>      }
>  
>      return NULL;
> @@ -1235,12 +1232,12 @@ static void qemu_tcg_init_vcpu(CPUState *cpu)
>  
>      tcg_cpu_address_space_init(cpu, cpu->as);
>  
> +    cpu->halt_cond = g_malloc0(sizeof(QemuCond));
> +    qemu_cond_init(cpu->halt_cond);
> +
>      /* share a single thread for all cpus with TCG */
>      if (!tcg_cpu_thread) {
>          cpu->thread = g_malloc0(sizeof(QemuThread));
> -        cpu->halt_cond = g_malloc0(sizeof(QemuCond));
> -        qemu_cond_init(cpu->halt_cond);
> -        tcg_halt_cond = cpu->halt_cond;
>          snprintf(thread_name, VCPU_THREAD_NAME_SIZE, "CPU %d/TCG",
>                   cpu->cpu_index);
>          qemu_thread_create(cpu->thread, thread_name, qemu_tcg_cpu_thread_fn,
> @@ -1254,7 +1251,6 @@ static void qemu_tcg_init_vcpu(CPUState *cpu)
>          tcg_cpu_thread = cpu->thread;
>      } else {
>          cpu->thread = tcg_cpu_thread;
> -        cpu->halt_cond = tcg_halt_cond;
>      }