Re: [PATCH v2 03/11] kvm-all: Do not allow reap vcpu dirty ring buffer if not ready

[Peter Xu]

Hi, Yong,

On Mon, Nov 21, 2022 at 11:26:35AM -0500, huangy81@chinatelecom.cn wrote:
> From: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> 
> When tested large vcpu size vm with dirtylimit feature, Qemu crashed
> due to the assertion in kvm_dirty_ring_reap_one, which assert that
> vcpu's kvm_dirty_gfns has been allocated and not NULL.
> 
> Because dirty ring reaper thread races with Qemu main thread, reaper
> may reap vcpu's dirty ring buffer when main thread doesn't complete
> vcpu instantiation. So add the waiting logic in reaper thread and
> start to reap until vcpu instantiation is completed.
> 
> Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> ---
>  accel/kvm/kvm-all.c | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index f99b0be..9457715 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -1401,6 +1401,35 @@ out:
>      kvm_slots_unlock();
>  }
>  
> +/*
> + * test if dirty ring has been initialized by checking if vcpu
> + * has been initialized and gfns was allocated correspondlingly.
> + * return true if dirty ring has been initialized, false otherwise.
> + */
> +static bool kvm_vcpu_dirty_ring_initialized(void)
> +{
> +    CPUState *cpu;
> +    MachineState *ms = MACHINE(qdev_get_machine());
> +    int ncpus = ms->smp.cpus;
> +
> +    /*
> +     * assume vcpu has not been initilaized if generation
> +     * id less than number of vcpu
> +     */
> +    if (ncpus > cpu_list_generation_id_get()) {
> +        return false;
> +    }
> +
> +    CPU_FOREACH(cpu) {
> +        if (!cpu->kvm_dirty_gfns) {
> +            return false;
> +        }
> +    }
> +
> +    return true;
> +}
> +
> +
>  static void *kvm_dirty_ring_reaper_thread(void *data)
>  {
>      KVMState *s = data;
> @@ -1410,6 +1439,13 @@ static void *kvm_dirty_ring_reaper_thread(void *data)
>  
>      trace_kvm_dirty_ring_reaper("init");
>  
> +retry:
> +    /* don't allow reaping dirty ring if ring buffer hasn't been mapped */
> +    if (!kvm_vcpu_dirty_ring_initialized()) {
> +        sleep(1);

The sleep here is probably not necessary.  Could you instead have a look at
the other much simpler patch?  Here:

https://lore.kernel.org/qemu-devel/20220927154653.77296-1-peterx@redhat.com/

> +        goto retry;
> +    }
> +
>      while (true) {
>          r->reaper_state = KVM_DIRTY_RING_REAPER_WAIT;
>          trace_kvm_dirty_ring_reaper("wait");
> -- 
> 1.8.3.1
> 
> 

-- 
Peter Xu