Re: [PATCH 5/5] core/cpu-common: initialise plugin state before thread creation

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 30/5/24 21:42, Alex Bennée wrote:
> Originally I tried to move where vCPU thread initialisation to later
> in realize. However pulling that thread (sic) got gnarly really
> quickly. It turns out some steps of CPU realization need values that
> can only be determined from the running vCPU thread.

FYI: 
https://lore.kernel.org/qemu-devel/20240528145953.65398-6-philmd@linaro.org/

> However having moved enough out of the thread creation we can now
> queue work before the thread starts (at least for TCG guests) and
> avoid the race between vcpu_init and other vcpu states a plugin might
> subscribe to.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>   hw/core/cpu-common.c | 20 ++++++++++++--------
>   1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/core/cpu-common.c b/hw/core/cpu-common.c
> index 6cfc01593a..bf1a7b8892 100644
> --- a/hw/core/cpu-common.c
> +++ b/hw/core/cpu-common.c
> @@ -222,14 +222,6 @@ static void cpu_common_realizefn(DeviceState *dev, Error **errp)
>           cpu_resume(cpu);
>       }
>   
> -    /* Plugin initialization must wait until the cpu start executing code */
> -#ifdef CONFIG_PLUGIN
> -    if (tcg_enabled()) {
> -        cpu->plugin_state = qemu_plugin_create_vcpu_state();
> -        async_run_on_cpu(cpu, qemu_plugin_vcpu_init__async, RUN_ON_CPU_NULL);
> -    }
> -#endif
> -
>       /* NOTE: latest generic point where the cpu is fully realized */
>   }
>   
> @@ -273,6 +265,18 @@ static void cpu_common_initfn(Object *obj)
>       QTAILQ_INIT(&cpu->watchpoints);
>   
>       cpu_exec_initfn(cpu);
> +
> +    /*
> +     * Plugin initialization must wait until the cpu start executing
> +     * code, but we must queue this work before the threads are
> +     * created to ensure we don't race.
> +     */
> +#ifdef CONFIG_PLUGIN
> +    if (tcg_enabled()) {
> +        cpu->plugin_state = qemu_plugin_create_vcpu_state();

Per https://etherpad.opendev.org/p/QEMU_vCPU_life, plugin_state could
be initialized in AccelCPUClass::cpu_instance_init (although this
callback is called at CPUClass::instance_post_init which I haven't
yet figured why).

> +        async_run_on_cpu(cpu, qemu_plugin_vcpu_init__async, RUN_ON_CPU_NULL);
> +    }
> +#endif
>   }
>   
>   static void cpu_common_finalize(Object *obj)