[PATCH RFC v5 03/13] vhost: batching fetches

Michael S. Tsirkin
2020-06-07T14:11:28Z

With this patch applied, new and old code perform identically.

Lots of extra optimizations are now possible, e.g.
we can fetch multiple heads with copy_from/to_user now.
We can get rid of maintaining the log array.  Etc etc.

Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: Eugenio P?rez <eperezma at redhat.com>
Link: https://lore.kernel.org/r/20200401183118.8334-4-eperezma at redhat.com
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---
 drivers/vhost/test.c  |  2 +-
 drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
 drivers/vhost/vhost.h |  3 +++
 3 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
index 9a3a09005e03..02806d6f84ef 100644
--- a/drivers/vhost/test.c
+++ b/drivers/vhost/test.c
@@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file
*f)
 	dev = &n->dev;
 	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
 	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
-	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
+	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
 		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);

 	f->private_data = n;
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 33a72edb3ccd..3b0609801381 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -304,6 +304,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 {
 	vq->num = 1;
 	vq->ndescs = 0;
+	vq->first_desc = 0;
 	vq->desc = NULL;
 	vq->avail = NULL;
 	vq->used = NULL;
@@ -372,6 +373,11 @@ static int vhost_worker(void *data)
 	return 0;
 }

+static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
+{
+	return vq->max_descs - UIO_MAXIOV;
+}
+
 static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
 {
 	kfree(vq->descs);
@@ -394,6 +400,9 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
 	for (i = 0; i < dev->nvqs; ++i) {
 		vq = dev->vqs[i];
 		vq->max_descs = dev->iov_limit;
+		if (vhost_vq_num_batch_descs(vq) < 0) {
+			return -EINVAL;
+		}
 		vq->descs = kmalloc_array(vq->max_descs,
 					  sizeof(*vq->descs),
 					  GFP_KERNEL);
@@ -1610,6 +1619,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int
ioctl, void __user *arg
 		vq->last_avail_idx = s.num;
 		/* Forget the cached index value. */
 		vq->avail_idx = vq->last_avail_idx;
+		vq->ndescs = vq->first_desc = 0;
 		break;
 	case VHOST_GET_VRING_BASE:
 		s.index = idx;
@@ -2179,7 +2189,7 @@ static int fetch_indirect_descs(struct vhost_virtqueue
*vq,
 	return 0;
 }

-static int fetch_descs(struct vhost_virtqueue *vq)
+static int fetch_buf(struct vhost_virtqueue *vq)
 {
 	unsigned int i, head, found = 0;
 	struct vhost_desc *last;
@@ -2192,7 +2202,11 @@ static int fetch_descs(struct vhost_virtqueue *vq)
 	/* Check it isn't doing very strange things with descriptor numbers. */
 	last_avail_idx = vq->last_avail_idx;

-	if (vq->avail_idx == vq->last_avail_idx) {
+	if (unlikely(vq->avail_idx == vq->last_avail_idx)) {
+		/* If we already have work to do, don't bother re-checking. */
+		if (likely(vq->ndescs))
+			return vq->num;
+
 		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
 			vq_err(vq, "Failed to access avail idx at %p\n",
 				&vq->avail->idx);
@@ -2283,6 +2297,24 @@ static int fetch_descs(struct vhost_virtqueue *vq)
 	return 0;
 }

+static int fetch_descs(struct vhost_virtqueue *vq)
+{
+	int ret = 0;
+
+	if (unlikely(vq->first_desc >= vq->ndescs)) {
+		vq->first_desc = 0;
+		vq->ndescs = 0;
+	}
+
+	if (vq->ndescs)
+		return 0;
+
+	while (!ret && vq->ndescs <= vhost_vq_num_batch_descs(vq))
+		ret = fetch_buf(vq);
+
+	return vq->ndescs ? 0 : ret;
+}
+
 /* This looks in the virtqueue and for the first available buffer, and converts
  * it to an iovec for convenient access.  Since descriptors consist of some
  * number of output then some number of input descriptors, it's actually
two
@@ -2308,7 +2340,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 	if (unlikely(log))
 		*log_num = 0;

-	for (i = 0; i < vq->ndescs; ++i) {
+	for (i = vq->first_desc; i < vq->ndescs; ++i) {
 		unsigned iov_count = *in_num + *out_num;
 		struct vhost_desc *desc = &vq->descs[i];
 		int access;
@@ -2354,14 +2386,19 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 		}

 		ret = desc->id;
+
+		if (!(desc->flags & VRING_DESC_F_NEXT))
+			break;
 	}

-	vq->ndescs = 0;
+	vq->first_desc = i + 1;

 	return ret;

 err:
-	vhost_discard_vq_desc(vq, 1);
+	for (i = vq->first_desc; i < vq->ndescs; ++i)
+		if (!(vq->descs[i].flags & VRING_DESC_F_NEXT))
+			vhost_discard_vq_desc(vq, 1);
 	vq->ndescs = 0;

 	return ret;
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 8a7b4191bc48..fed36af5c444 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -81,6 +81,7 @@ struct vhost_virtqueue {

 	struct vhost_desc *descs;
 	int ndescs;
+	int first_desc;
 	int max_descs;

 	struct file *kick;
@@ -257,6 +258,8 @@ static inline void vhost_vq_set_backend(struct
vhost_virtqueue *vq,
 					void *private_data)
 {
 	vq->private_data = private_data;
+	vq->ndescs = 0;
+	vq->first_desc = 0;
 }

 /**
--
MST