Re: [PATCH] qemu-kvm: introduce cpu_start/cpu_stop commands

[Gleb Natapov <gleb@redhat.com>]

On Mon, Nov 22, 2010 at 05:00:18PM -0600, Anthony Liguori wrote:
> qemu-kvm vcpu threads don't response to SIGSTOP/SIGCONT.  Instead of teaching
> them to respond to these signals, introduce monitor commands that stop and start
> individual vcpus.
> 
> The purpose of these commands are to implement CPU hard limits using an external
> tool that watches the CPU consumption and stops the CPU as appropriate.
> 
> The monitor commands provide a more elegant solution that signals because it
> ensures that a stopped vcpu isn't holding the qemu_mutex.
> 
Do you really want to stop vcpu while it holds guest lock? Does external tool
have enough info to make smart decision about how to limit vcpu runtime.

> I'll reply to this note with an example tool.
> 
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
> 
> diff --git a/hmp-commands.hx b/hmp-commands.hx
> index ba6de28..827bd67 100644
> --- a/hmp-commands.hx
> +++ b/hmp-commands.hx
> @@ -279,6 +279,24 @@ Resume emulation.
>  ETEXI
>  
>      {
> +        .name       = "cpu_start",
> +        .args_type  = "cpu:i",
> +        .params     = "[cpu]",
> +        .help       = "start cpu emulation",
> +        .user_print = monitor_user_noop,
> +        .mhandler.cmd_new = do_vcpu_start,
> +    },
> +
> +    {
> +        .name       = "cpu_stop",
> +        .args_type  = "cpu:i",
> +        .params     = "[cpu]",
> +        .help       = "stop cpu emulation",
> +        .user_print = monitor_user_noop,
> +        .mhandler.cmd_new = do_vcpu_stop,
> +    },
> +
> +    {
>          .name       = "gdbserver",
>          .args_type  = "device:s?",
>          .params     = "[device]",
> diff --git a/qemu-kvm.c b/qemu-kvm.c
> index 471306b..35121ed 100644
> --- a/qemu-kvm.c
> +++ b/qemu-kvm.c
> @@ -1351,6 +1351,65 @@ static void pause_all_threads(void)
>      }
>  }
>  
> +static void vcpu_stop(int cpu)
> +{
> +    CPUState *env = first_cpu;
> +
> +    for (env = first_cpu; env; env = env->next_cpu) {
> +        if (env->cpu_index == cpu) {
> +            break;
> +        }
> +    }
> +
> +    if (env) {
> +        if (env != cpu_single_env) {
> +            env->stop = 1;
> +            pthread_kill(env->kvm_cpu_state.thread, SIG_IPI);
> +        } else {
> +            env->stop = 0;
> +            env->stopped = 1;
> +            cpu_exit(env);
> +        }
> +
> +        while (!env->stopped) {
> +            qemu_cond_wait(&qemu_pause_cond);
> +        }
> +    }
> +}
> +
> +static void vcpu_start(int cpu)
> +{
> +    CPUState *env = first_cpu;
> +
> +    assert(!cpu_single_env);
> +
> +    for (env = first_cpu; env; env = env->next_cpu) {
> +        if (env->cpu_index == cpu) {
> +            break;
> +        }
> +    }
> +
> +    if (env) {
> +        env->stop = 0;
> +        env->stopped = 0;
> +        pthread_kill(env->kvm_cpu_state.thread, SIG_IPI);
> +    }
> +}
> +
> +int do_vcpu_stop(Monitor *mon, const QDict *qdict, QObject **ret_data)
> +{
> +    int vcpu = qdict_get_int(qdict, "cpu");
> +    vcpu_stop(vcpu);
> +    return 0;
> +}
> +
> +int do_vcpu_start(Monitor *mon, const QDict *qdict, QObject **ret_data)
> +{
> +    int vcpu = qdict_get_int(qdict, "cpu");
> +    vcpu_start(vcpu);
> +    return 0;
> +}
> +
>  static void resume_all_threads(void)
>  {
>      CPUState *penv = first_cpu;
> diff --git a/sysemu.h b/sysemu.h
> index 849dc8c..3ef68dd 100644
> --- a/sysemu.h
> +++ b/sysemu.h
> @@ -61,6 +61,9 @@ void qemu_system_reset(void);
>  void qemu_add_exit_notifier(Notifier *notify);
>  void qemu_remove_exit_notifier(Notifier *notify);
>  
> +int do_vcpu_stop(Monitor *mon, const QDict *qdict, QObject **ret_data);
> +int do_vcpu_start(Monitor *mon, const QDict *qdict, QObject **ret_data);
> +
>  void do_savevm(Monitor *mon, const QDict *qdict);
>  int load_vmstate(const char *name);
>  void do_delvm(Monitor *mon, const QDict *qdict);
> -- 
> 1.7.0.4
> 
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--
			Gleb.