Re: [Qemu-devel] [PATCH] Add option to slow down qemu

[Stefan Weil <sw@weilnetz.de>]

Am 11.01.2013 14:40, schrieb Wolfgang Mauerer:
> For slow targets and fast hosts, the emulation may be faster
> than the actual hardware, which can be undesirable for various
> reasons. Add a run-time option to slow down the emulation
> by sleeping in the CPU emulation.
>
> Signed-off-by: Wolfgang Mauerer<wolfgang.mauerer@siemens.com>
> ---
>   cpus.c                |   34 ++++++++++++++++++++++++++++++++++
>   include/qemu-common.h |    2 ++
>   qemu-options.hx       |   13 +++++++++++++
>   vl.c                  |   10 ++++++++++
>   4 files changed, 59 insertions(+), 0 deletions(-)
>
> diff --git a/cpus.c b/cpus.c
> index 4a7782a..41a9e0c 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -106,6 +106,11 @@ static QEMUTimer *icount_warp_timer;
>   static int64_t vm_clock_warp_start;
>   static int64_t qemu_icount;
>
> +static double slowdown_factor = 0.0;
> +#ifndef _WIN32
> +static struct timespec slowdown_delay;
> +#endif
> +
>    

Hi,

slowdown_delay is used in configure_slowdown unconditionally,
so I don't expect that the _WIN32 case will compile.

What about using g_usleep? It avoids the conditional compilation.

Is the comparison of double value "slowdown_factor" with 0.0
time critical, or do all QEMU platforms have a fast FPU?

Setting a boolean value once and testing that value would
be much faster of course.

Cheers,
Stefan W.


>   typedef struct TimersState {
>       int64_t cpu_ticks_prev;
>       int64_t cpu_ticks_offset;
> @@ -385,6 +390,21 @@ void configure_icount(const char *option)
>                      qemu_get_clock_ns(vm_clock) + get_ticks_per_sec() / 10);
>   }
>
> +void configure_slowdown(const char *option)
> +{
> +    if (!option) {
> +        return;
> +    }
> +
> +    slowdown_factor = strtod(option, NULL)/100.0;
> +    /* We cannot provide speedups, obviously */
> +    if (slowdown_factor<  0) {
> +        slowdown_factor *= -1.0;
> +    }
> +
> +    slowdown_delay.tv_sec = 0;
> +}
> +
>   /***********************************************************/
>   void hw_error(const char *fmt, ...)
>   {
> @@ -1092,6 +1112,7 @@ void vm_stop_force_state(RunState state)
>   static int tcg_cpu_exec(CPUArchState *env)
>   {
>       int ret;
> +    int64_t ss, delta;
>   #ifdef CONFIG_PROFILER
>       int64_t ti;
>   #endif
> @@ -1112,7 +1133,20 @@ static int tcg_cpu_exec(CPUArchState *env)
>           env->icount_decr.u16.low = decr;
>           env->icount_extra = count;
>       }
> +    ss = qemu_get_clock_ns(rt_clock);
> +
>       ret = cpu_exec(env);
> +
> +    if (slowdown_factor>  0) {
> +        delta = qemu_get_clock_ns(rt_clock) - ss;
> +        delta *= slowdown_factor;
> +#ifdef _WIN32
> +        Sleep(delta/1000000l);
> +#else
> +        slowdown_delay.tv_nsec = delta;
> +        nanosleep(&slowdown_delay, NULL);
> +#endif
> +    }
>   #ifdef CONFIG_PROFILER
>       qemu_time += profile_getclock() - ti;
>   #endif
> diff --git a/include/qemu-common.h b/include/qemu-common.h
> index ca464bb..652abb9 100644
> --- a/include/qemu-common.h
> +++ b/include/qemu-common.h
> @@ -119,6 +119,8 @@ static inline char *realpath(const char *path, char *resolved_path)
>   void configure_icount(const char *option);
>   extern int use_icount;
>
> +void configure_slowdown(const char *option);
> +
>   /* FIXME: Remove NEED_CPU_H.  */
>   #ifndef NEED_CPU_H
>
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 9df0cde..2f6580f 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -2665,6 +2665,19 @@ order cores with complex cache hierarchies.  The number of instructions
>   executed often has little or no correlation with actual performance.
>   ETEXI
>
> +DEF("slowdown", HAS_ARG, QEMU_OPTION_slowdown, \
> +    "-slowdown s \n" \
> +    "                Slow down the CPU emulation by (approximately) s"
> +    "		     percent\n", QEMU_ARCH_ALL)
> +STEXI
> +@item -slowdown [@var{s}]
> +@findex -slowdown
> +Slow down the virtual CPU by approximately s percent. This makes it
> +possible to align the emulated machine's performance roughly with
> +the performance of physical entities, but does not provide identical
> +performance profiles since the emulation is not cycle accurate.
> +ETEXI
> +
>   DEF("watchdog", HAS_ARG, QEMU_OPTION_watchdog, \
>       "-watchdog i6300esb|ib700\n" \
>       "                enable virtual hardware watchdog [default=none]\n",
> diff --git a/vl.c b/vl.c
> index 79e5122..624551f 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -2524,6 +2524,7 @@ int main(int argc, char **argv, char **envp)
>       int i;
>       int snapshot, linux_boot;
>       const char *icount_option = NULL;
> +    const char *slowdown_option = NULL;
>       const char *initrd_filename;
>       const char *kernel_filename, *kernel_cmdline;
>       char boot_devices[33] = "cad"; /* default to HD->floppy->CD-ROM */
> @@ -3402,6 +3403,9 @@ int main(int argc, char **argv, char **envp)
>               case QEMU_OPTION_icount:
>                   icount_option = optarg;
>                   break;
> +            case QEMU_OPTION_slowdown:
> +                slowdown_option = optarg;
> +                break;
>               case QEMU_OPTION_incoming:
>                   incoming = optarg;
>                   runstate_set(RUN_STATE_INMIGRATE);
> @@ -3765,6 +3769,12 @@ int main(int argc, char **argv, char **envp)
>       /* clean up network at qemu process termination */
>       atexit(&net_cleanup);
>
> +    if (slowdown_option&&  (kvm_enabled() || xen_enabled())) {
> +        fprintf(stderr, "-slowdown is not allowed with kvm or xen\n");
> +        exit(1);
> +    }
> +    configure_slowdown(slowdown_option);
> +
>       if (net_init_clients()<  0) {
>           exit(1);
>       }
>