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From:   Guo Zhi <qtxuning1999@sjtu.edu.cn>
To:     eperezma@redhat.com, jasowang@redhat.com, sgarzare@redhat.com,
        mst@redhat.com
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
        Guo Zhi <qtxuning1999@sjtu.edu.cn>
Subject: [RFC v3 6/7] virtio: in order support for virtio_ring
Date:   Thu,  1 Sep 2022 13:54:33 +0800
Message-Id: <20220901055434.824-7-qtxuning1999@sjtu.edu.cn>
In-Reply-To: <20220901055434.824-1-qtxuning1999@sjtu.edu.cn>
References: <20220901055434.824-1-qtxuning1999@sjtu.edu.cn>
Precedence: bulk

If in order feature negotiated, we can skip the used ring to get
buffer's desc id sequentially.  For skipped buffers in the batch, the
used ring doesn't contain the buffer length, actually there is not need
to get skipped buffers' length as they are tx buffer.

Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
---
 drivers/virtio/virtio_ring.c | 74 +++++++++++++++++++++++++++++++-----
 1 file changed, 64 insertions(+), 10 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 00aa4b7a49c2..d52624179b43 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -103,6 +103,9 @@ struct vring_virtqueue {
 	/* Host supports indirect buffers */
 	bool indirect;
 
+	/* Host supports in order feature */
+	bool in_order;
+
 	/* Host publishes avail event idx */
 	bool event;
 
@@ -144,6 +147,19 @@ struct vring_virtqueue {
 			/* DMA address and size information */
 			dma_addr_t queue_dma_addr;
 			size_t queue_size_in_bytes;
+
+			/* If in_order feature is negotiated, here is the next head to consume */
+			u16 next_desc_begin;
+			/*
+			 * If in_order feature is negotiated,
+			 * here is the last descriptor's id in the batch
+			 */
+			u16 last_desc_in_batch;
+			/*
+			 * If in_order feature is negotiated,
+			 * buffers except last buffer in the batch are skipped buffer
+			 */
+			bool is_skipped_buffer;
 		} split;
 
 		/* Available for packed ring */
@@ -584,8 +600,6 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 					 total_sg * sizeof(struct vring_desc),
 					 VRING_DESC_F_INDIRECT,
 					 false);
-		vq->split.desc_extra[head & (vq->split.vring.num - 1)].flags &=
-			~VRING_DESC_F_NEXT;
 	}
 
 	/* We're using some buffers from the free list. */
@@ -701,8 +715,16 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
 	}
 
 	vring_unmap_one_split(vq, i);
-	vq->split.desc_extra[i].next = vq->free_head;
-	vq->free_head = head;
+	/*
+	 * If in_order feature is negotiated,
+	 * the descriptors are made available in order.
+	 * Since the free_head is already a circular list,
+	 * it must consume it sequentially.
+	 */
+	if (!vq->in_order) {
+		vq->split.desc_extra[i].next = vq->free_head;
+		vq->free_head = head;
+	}
 
 	/* Plus final descriptor */
 	vq->vq.num_free++;
@@ -744,7 +766,7 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 	void *ret;
-	unsigned int i;
+	unsigned int i, j;
 	u16 last_used;
 
 	START_USE(vq);
@@ -763,11 +785,38 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
 	/* Only get used array entries after they have been exposed by host. */
 	virtio_rmb(vq->weak_barriers);
 
-	last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
-	i = virtio32_to_cpu(_vq->vdev,
-			vq->split.vring.used->ring[last_used].id);
-	*len = virtio32_to_cpu(_vq->vdev,
-			vq->split.vring.used->ring[last_used].len);
+	if (vq->in_order) {
+		last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
+		if (!vq->split.is_skipped_buffer) {
+			vq->split.last_desc_in_batch =
+				virtio32_to_cpu(_vq->vdev,
+						vq->split.vring.used->ring[last_used].id);
+			vq->split.is_skipped_buffer = true;
+		}
+		/* For skipped buffers in batch, we can ignore the len info, simply set len as 0 */
+		if (vq->split.next_desc_begin != vq->split.last_desc_in_batch) {
+			*len = 0;
+		} else {
+			*len = virtio32_to_cpu(_vq->vdev,
+					       vq->split.vring.used->ring[last_used].len);
+			vq->split.is_skipped_buffer = false;
+		}
+		i = vq->split.next_desc_begin;
+		j = i;
+		/* Indirect only takes one descriptor in descriptor table */
+		while (!vq->indirect && (vq->split.desc_extra[j].flags & VRING_DESC_F_NEXT))
+			j = (j + 1) & (vq->split.vring.num - 1);
+		/* move to next */
+		j = (j + 1) % vq->split.vring.num;
+		/* Next buffer will use this descriptor in order */
+		vq->split.next_desc_begin = j;
+	} else {
+		last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
+		i = virtio32_to_cpu(_vq->vdev,
+				    vq->split.vring.used->ring[last_used].id);
+		*len = virtio32_to_cpu(_vq->vdev,
+				       vq->split.vring.used->ring[last_used].len);
+	}
 
 	if (unlikely(i >= vq->split.vring.num)) {
 		BAD_RING(vq, "id %u out of range\n", i);
@@ -2223,6 +2272,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
 
 	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
 		!context;
+	vq->in_order = virtio_has_feature(vdev, VIRTIO_F_IN_ORDER);
 	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
 
 	if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
@@ -2235,6 +2285,10 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
 	vq->split.avail_flags_shadow = 0;
 	vq->split.avail_idx_shadow = 0;
 
+	vq->split.next_desc_begin = 0;
+	vq->split.last_desc_in_batch = 0;
+	vq->split.is_skipped_buffer = false;
+
 	/* No callback?  Tell other side not to bother us. */
 	if (!callback) {
 		vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
-- 
2.17.1