Re: [RFC 4/5] virtio: get desc id in order

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在 2022/7/21 16:43, Guo Zhi 写道:
If in order feature negotiated, we can skip the used ring to get
buffer's desc id sequentially.


Let's rename the patch to something like "in order support for virtio_ring"



Signed-off-by: Guo Zhi <qtxuning1999@xxxxxxxxxxx>
---
  drivers/virtio/virtio_ring.c | 37 ++++++++++++++++++++++++++++--------
  1 file changed, 29 insertions(+), 8 deletions(-)


I don't see packed support in this patch, we need to implement that.



diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index a5ec724c0..4d57a4edc 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -144,6 +144,9 @@ struct vring_virtqueue {
  			/* DMA address and size information */
  			dma_addr_t queue_dma_addr;
  			size_t queue_size_in_bytes;
+
+			/* In order feature batch begin here */
+			u16 next_batch_desc_begin;
  		} split;
/* Available for packed ring */
@@ -700,8 +703,10 @@ 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 (!virtio_has_feature(vq->vq.vdev, VIRTIO_F_IN_ORDER)) {
+		vq->split.desc_extra[i].next = vq->free_head;
+		vq->free_head = head;
+	}


Let's add a comment to explain why we don't need anything if in order is neogitated.


/* Plus final descriptor */
  	vq->vq.num_free++;
@@ -743,7 +748,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
  {
  	struct vring_virtqueue *vq = to_vvq(_vq);
  	void *ret;
-	unsigned int i;
+	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
+	unsigned int i, j;
  	u16 last_used;
START_USE(vq);
@@ -762,11 +768,24 @@ 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 (virtio_has_feature(_vq->vdev, VIRTIO_F_IN_ORDER)) {
+		/* Skip used ring and get used desc in order*/
+		i = vq->split.next_batch_desc_begin;
+		j = i;
+		while (vq->split.vring.desc[j].flags & nextflag)


Let's don't depend on the descriptor ring which is under the control of the malicious hypervisor.

Let's use desc_extra that is not visible by the hypervisor. More can be seen in this commit:

72b5e8958738 ("virtio-ring: store DMA metadata in desc_extra for split virtqueue")


+			j = (j + 1) % vq->split.vring.num;
+		/* move to next */
+		j = (j + 1) % vq->split.vring.num;
+		vq->split.next_batch_desc_begin = j;


I'm not sure I get the logic here, basically I think we should check buffer instead of descriptor here.

So if vring.used->ring[last_used].id != last_used, we know all [last_used, vring.used->ring[last_used].id] have been used in a batch?


+
+		/* TODO: len of buffer */


So spec said:

"

The skipped buffers (for which no used ring entry was written) are assumed to have been used (read or written) by the device completely.


"

Thanks


+	} 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);
@@ -2234,6 +2253,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
  	vq->split.avail_flags_shadow = 0;
  	vq->split.avail_idx_shadow = 0;
+ vq->split.next_batch_desc_begin = 0;
+
  	/* No callback?  Tell other side not to bother us. */
  	if (!callback) {
  		vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;




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