linux-kernel - Re: [RFC V5 PATCH 8/8] vhost: event suppression for packed ring

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Message-ID: <20180603154004.GA24012@wei-ubt>
Date:   Sun, 3 Jun 2018 23:40:04 +0800
From:   Wei Xu <wexu@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     mst@...hat.com, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, jfreimann@...hat.com,
        tiwei.bie@...el.com
Subject: Re: [RFC V5 PATCH 8/8] vhost: event suppression for packed ring

On Thu, May 31, 2018 at 11:09:07AM +0800, Jason Wang wrote:
> 
> 
> 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.

OK, thanks.

> 
> Thanks