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Subject: Re: [RFC V5 PATCH 8/8] vhost: event suppression for packed ring
To:     Wei Xu <wexu@redhat.com>
Cc:     mst@redhat.com, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, jfreimann@redhat.com,
        tiwei.bie@intel.com
References: <1527559830-8133-1-git-send-email-jasowang@redhat.com>
 <1527559830-8133-9-git-send-email-jasowang@redhat.com>
 <20180530114200.GA23792@wei-ubt>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <12f2c455-5868-3b07-0eba-d49dcafd10f2@redhat.com>
Date:   Thu, 31 May 2018 11:09:07 +0800
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On 2018年05月30日 19:42, Wei Xu wrote:
>>   /* This actually signals the guest, using eventfd. */
>>   void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>>   {
>> @@ -2802,10 +2930,34 @@ static bool vhost_enable_notify_packed(struct vhost_dev *dev,
>>   				       struct vhost_virtqueue *vq)
>>   {
>>   	struct vring_desc_packed *d = vq->desc_packed + vq->avail_idx;
>> -	__virtio16 flags;
>> +	__virtio16 flags = RING_EVENT_FLAGS_ENABLE;
>>   	int ret;
>>   
>> -	/* FIXME: disable notification through device area */
>> +	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
>> +		return false;
>> +	vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;
> 'used_flags' was originally designed for 1.0, why should we pay attetion to it here?
>
> Wei

It was used to recored whether or not we've disabled notification. Then 
we can avoid unnecessary userspace writes or memory barriers.

Thanks