Re: [PATCH 1/1] allow using a higher icount

[Elisha Hollander]

Oh nice, I didn't know that

On Tue, Aug 27, 2024, 12:39 Alex Bennée <alex.bennee@linaro.org> wrote:

Elisha Hollander <just4now666666@gmail.com> writes:

> Signed-off-by: Elisha Hollander <just4now666666@gmail.com>

What is the use-case for this patch?

If you are simply looking to slow the emulated system down please have a
look at:

  https://qemu.readthedocs.io/en/master/about/emulation.html#limit-instructions-per-second

which uses the plugin system to limit the run rate and sleep if its
running too fast. The longer term goal is to deprecate the icount clock
alignment feature from the core code and leave icount to just provide
the deterministic execution needed for record/replay and reverse
debugging.


> ---
>  accel/tcg/cpu-exec.c      | 4 +---
>  accel/tcg/icount-common.c | 4 ++--
>  2 files changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 8163295f34..4c2baf8ed4 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -95,11 +95,10 @@ static void align_clocks(SyncClocks *sc, CPUState *cpu)
>  static void print_delay(const SyncClocks *sc)
>  {
>      static float threshold_delay;
> -    static int64_t last_realtime_clock;
>      static int nb_prints;
> 
>      if (icount_align_option &&
> -        sc->realtime_clock - last_realtime_clock >= MAX_DELAY_PRINT_RATE &&
> +        sc->diff_clk >= MAX_DELAY_PRINT_RATE &&
>          nb_prints < MAX_NB_PRINTS) {
>          if ((-sc->diff_clk / (float)1000000000LL > threshold_delay) ||
>              (-sc->diff_clk / (float)1000000000LL <
> @@ -109,7 +108,6 @@ static void print_delay(const SyncClocks *sc)
>                          threshold_delay - 1,
>                          threshold_delay);
>              nb_prints++;
> -            last_realtime_clock = sc->realtime_clock;
>          }
>      }
>  }
> diff --git a/accel/tcg/icount-common.c b/accel/tcg/icount-common.c
> index 8d3d3a7e9d..f07f8baf4d 100644
> --- a/accel/tcg/icount-common.c
> +++ b/accel/tcg/icount-common.c
> @@ -46,8 +46,8 @@
>   * is TCG-specific, and does not need to be built for other accels.
>   */
>  static bool icount_sleep = true;
> -/* Arbitrarily pick 1MIPS as the minimum allowable speed.  */
> -#define MAX_ICOUNT_SHIFT 10
> +/* Arbitrarily pick the minimum allowable speed.  */
> +#define MAX_ICOUNT_SHIFT 30
> 
>  /* Do not count executed instructions */
>  ICountMode use_icount = ICOUNT_DISABLED;

--
Alex Bennée
Virtualisation Tech Lead @ Linaro