vhost: cache used event for better performance

[mirror_ubuntu-zesty-kernel.git] / drivers / vhost / vhost.c

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 2f58181b918d350968951c8b18f35b106887cb84..c0f81e8cb50c8bc330f3572ab51eb6e3e4629950 100644 (file)
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -290,6 +290,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
        vq->avail = NULL;
        vq->used = NULL;
        vq->last_avail_idx = 0;
+       vq->last_used_event = 0;
        vq->avail_idx = 0;
        vq->last_used_idx = 0;
        vq->signalled_used = 0;
@@ -1324,7 +1325,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
                        r = -EINVAL;
                        break;
                }
-               vq->last_avail_idx = s.num;
+               vq->last_avail_idx = vq->last_used_event = s.num;
                /* Forget the cached index value. */
                vq->avail_idx = vq->last_avail_idx;
                break;
@@ -2159,10 +2160,6 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
        __u16 old, new;
        __virtio16 event;
        bool v;
-       /* Flush out used index updates. This is paired
-        * with the barrier that the Guest executes when enabling
-        * interrupts. */
-       smp_mb();
 
        if (vhost_has_feature(vq, VIRTIO_F_NOTIFY_ON_EMPTY) &&
            unlikely(vq->avail_idx == vq->last_avail_idx))
@@ -2170,6 +2167,10 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 
        if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
                __virtio16 flags;
+               /* Flush out used index updates. This is paired
+                * with the barrier that the Guest executes when enabling
+                * interrupts. */
+               smp_mb();
                if (vhost_get_user(vq, flags, &vq->avail->flags)) {
                        vq_err(vq, "Failed to get flags");
                        return true;
@@ -2184,11 +2185,26 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
        if (unlikely(!v))
                return true;
 
+       /* We're sure if the following conditions are met, there's no
+        * need to notify guest:
+        * 1) cached used event is ahead of new
+        * 2) old to new updating does not cross cached used event. */
+       if (vring_need_event(vq->last_used_event, new + vq->num, new) &&
+           !vring_need_event(vq->last_used_event, new, old))
+               return false;
+
+       /* Flush out used index updates. This is paired
+        * with the barrier that the Guest executes when enabling
+        * interrupts. */
+       smp_mb();
+
        if (vhost_get_user(vq, event, vhost_used_event(vq))) {
                vq_err(vq, "Failed to get used event idx");
                return true;
        }
-       return vring_need_event(vhost16_to_cpu(vq, event), new, old);
+       vq->last_used_event = vhost16_to_cpu(vq, event);
+
+       return vring_need_event(vq->last_used_event, new, old);
 }
 
 /* This actually signals the guest, using eventfd. */