[SRU][Zesty][PATCH 1/1] Revert "vhost: cache used event for better performance"

Joseph Salisbury joseph.salisbury at canonical.com
Fri Aug 25 16:07:55 UTC 2017


BugLink: http://bugs.launchpad.net/bugs/1711251

This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92.
---
 drivers/vhost/vhost.c | 28 ++++++----------------------
 drivers/vhost/vhost.h |  3 ---
 2 files changed, 6 insertions(+), 25 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 8f99fe0..8d05a40 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -290,7 +290,6 @@ 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;
@@ -1325,7 +1324,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
 			r = -EINVAL;
 			break;
 		}
-		vq->last_avail_idx = vq->last_used_event = s.num;
+		vq->last_avail_idx = s.num;
 		/* Forget the cached index value. */
 		vq->avail_idx = vq->last_avail_idx;
 		break;
@@ -2157,6 +2156,10 @@ 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))
@@ -2164,10 +2167,6 @@ 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;
@@ -2182,26 +2181,11 @@ 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;
 	}
-	vq->last_used_event = vhost16_to_cpu(vq, event);
-
-	return vring_need_event(vq->last_used_event, new, old);
+	return vring_need_event(vhost16_to_cpu(vq, event), new, old);
 }
 
 /* This actually signals the guest, using eventfd. */
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index a9cbbb1..78f3c5f 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -107,9 +107,6 @@ struct vhost_virtqueue {
 	/* Last index we used. */
 	u16 last_used_idx;
 
-	/* Last used evet we've seen */
-	u16 last_used_event;
-
 	/* Used flags */
 	u16 used_flags;
 
-- 
2.7.4





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