[PATCH net-next RFC 2/5] vhost: introduce helper to prefetch desc index

[Jason Wang]

This patch introduces vhost_prefetch_desc_indices() which could batch
descriptor indices fetching and used ring updating. This intends to
reduce the cache misses of indices fetching and updating and reduce
cache line bounce when virtqueue is almost full. copy_to_user() was
used in order to benefit from modern cpus that support fast string
copy. Batched virtqueue processing will be the first user.

Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
 drivers/vhost/vhost.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++
 drivers/vhost/vhost.h |  3 +++
 2 files changed, 58 insertions(+)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index f87ec75..8424166d 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2437,6 +2437,61 @@ struct vhost_msg_node *vhost_dequeue_msg(struct vhost_dev *dev,
 }
 EXPORT_SYMBOL_GPL(vhost_dequeue_msg);
 
+int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
+				struct vring_used_elem *heads,
+				u16 num, bool used_update)
+{
+	int ret, ret2;
+	u16 last_avail_idx, last_used_idx, total, copied;
+	__virtio16 avail_idx;
+	struct vring_used_elem __user *used;
+	int i;
+
+	if (unlikely(vhost_get_avail(vq, avail_idx, &vq->avail->idx))) {
+		vq_err(vq, "Failed to access avail idx at %p\n",
+		       &vq->avail->idx);
+		return -EFAULT;
+	}
+	last_avail_idx = vq->last_avail_idx & (vq->num - 1);
+	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
+	total = vq->avail_idx - vq->last_avail_idx;
+	ret = total = min(total, num);
+
+	for (i = 0; i < ret; i++) {
+		ret2 = vhost_get_avail(vq, heads[i].id,
+				      &vq->avail->ring[last_avail_idx]);
+		if (unlikely(ret2)) {
+			vq_err(vq, "Failed to get descriptors\n");
+			return -EFAULT;
+		}
+		last_avail_idx = (last_avail_idx + 1) & (vq->num - 1);
+	}
+
+	if (!used_update)
+		return ret;
+
+	last_used_idx = vq->last_used_idx & (vq->num - 1);
+	while (total) {
+		copied = min((u16)(vq->num - last_used_idx), total);
+		ret2 = vhost_copy_to_user(vq,
+					  &vq->used->ring[last_used_idx],
+					  &heads[ret - total],
+					  copied * sizeof(*used));
+
+		if (unlikely(ret2)) {
+			vq_err(vq, "Failed to update used ring!\n");
+			return -EFAULT;
+		}
+
+		last_used_idx = 0;
+		total -= copied;
+	}
+
+	/* Only get avail ring entries after they have been exposed by guest. */
+	smp_rmb();
+	return ret;
+}
+EXPORT_SYMBOL(vhost_prefetch_desc_indices);
 
 static int __init vhost_init(void)
 {
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 39ff897..16c2cb6 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -228,6 +228,9 @@ ssize_t vhost_chr_read_iter(struct vhost_dev *dev, struct iov_iter *to,
 ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
 			     struct iov_iter *from);
 int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
+int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
+				struct vring_used_elem *heads,
+				u16 num, bool used_update);
 
 #define vq_err(vq, fmt, ...) do {                                  \
 		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
-- 
2.7.4