Re: [Qemu-devel] [RFC v3 12/19] tcg: add kick timer for single-threaded vCPU emulation

[Sergey Fedorov]

On 03/06/16 23:40, Alex Bennée wrote:
> diff --git a/cpus.c b/cpus.c
> index 1694ce9..12e04c9 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -1208,9 +1208,29 @@ static int tcg_cpu_exec(CPUState *cpu)
>      return ret;
>  }
>  
> +/* Single-threaded TCG
> + *
> + * In the single-threaded case each vCPU is simulated in turn. If
> + * there is more than a single vCPU we create a simple timer to kick
> + * the vCPU and ensure we don't get stuck in a tight loop in one vCPU.
> + * This is done explicitly rather than relying on side-effects
> + * elsewhere.
> + */
> +static void qemu_cpu_kick_no_halt(void);
> +#define TCG_KICK_FREQ (qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + \
> +                       NANOSECONDS_PER_SECOND / 10)

Hmm, it doesn't look nice to wrap calculation of the next timeout in a
macro and name it '*_FREQ'. I think we'd better do like this:

#define TCG_KICK_PERIOD (NANOSECONDS_PER_SECOND / 10)

static inline int64_t qemu_tcg_next_kick(void)
{
    return qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + TCG_KICK_PERIOD;
}

and use it like this:

timer_mod(kick_timer, qemu_tcg_next_kick());

Kind regards,
Sergey

> +
> +static void kick_tcg_thread(void *opaque)
> +{
> +    QEMUTimer *self = *(QEMUTimer **) opaque;
> +    timer_mod(self, TCG_KICK_FREQ);
> +    qemu_cpu_kick_no_halt();
> +}
> +
>  static void *qemu_tcg_cpu_thread_fn(void *arg)
>  {
>      CPUState *cpu = arg;
> +    QEMUTimer *kick_timer;
>  
>      rcu_register_thread();
>  
> @@ -1234,6 +1254,13 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
>          }
>      }
>  
> +    /* Set to kick if we have to do more than one vCPU */
> +    if (CPU_NEXT(first_cpu)) {
> +        kick_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,  kick_tcg_thread,
> +                                  &kick_timer);
> +        timer_mod(kick_timer, TCG_KICK_FREQ);
> +    }
> +
>      /* process any pending work */
>      atomic_mb_set(&exit_request, 1);
>