Re: [RFC PATCH v9 11/23] vhost: Move last chain id to SVQ element

[Jason Wang]

在 2022/7/7 02:39, Eugenio Pérez 写道:
We will allow SVQ user to store opaque data for each element, so its
easier if we store this kind of information just at avail.

Signed-off-by: Eugenio Pérez <epere...@redhat.com>


Note that kernel driver doesn't have this optimization so far.  I wonder 
if this is not a must, let's post this on top of the shadow CVQ stuffs.

Thanks


---
  hw/virtio/vhost-shadow-virtqueue.h |  3 +++
  hw/virtio/vhost-shadow-virtqueue.c | 14 ++++++++------
  2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.h 
b/hw/virtio/vhost-shadow-virtqueue.h
index e434dc63b0..0e434e9fd0 100644
--- a/hw/virtio/vhost-shadow-virtqueue.h
+++ b/hw/virtio/vhost-shadow-virtqueue.h
@@ -17,6 +17,9 @@
  
  typedef struct SVQElement {
      VirtQueueElement *elem;
+
+    /* Last descriptor of the chain */
+    uint32_t last_chain_id;
  } SVQElement;
  
  typedef struct VhostShadowVirtqueue VhostShadowVirtqueue;
diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
b/hw/virtio/vhost-shadow-virtqueue.c
index cf1745fd4d..c5e49e51c5 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -239,7 +239,9 @@ static bool vhost_svq_add(VhostShadowVirtqueue *svq, const 
struct iovec *out_sg,
                            size_t out_num, const struct iovec *in_sg,
                            size_t in_num, VirtQueueElement *elem)
  {
+    SVQElement *svq_elem;
      unsigned qemu_head;
+    size_t n;
      bool ok = vhost_svq_add_split(svq, out_sg, out_num, in_sg, in_num,
                                    &qemu_head);
      if (unlikely(!ok)) {
@@ -247,7 +249,10 @@ static bool vhost_svq_add(VhostShadowVirtqueue *svq, const 
struct iovec *out_sg,
          return false;
      }
  
-    svq->ring_id_maps[qemu_head].elem = elem;
+    n = out_num + in_num;
+    svq_elem = &svq->ring_id_maps[qemu_head];
+    svq_elem->elem = elem;
+    svq_elem->last_chain_id = vhost_svq_last_desc_of_chain(svq, n, qemu_head);
      return true;
  }
  
@@ -400,7 +405,7 @@ static SVQElement vhost_svq_get_buf(VhostShadowVirtqueue *svq, uint32_t *len)
      const vring_used_t *used = svq->vring.used;
      vring_used_elem_t used_elem;
      SVQElement svq_elem = vhost_svq_empty_elem();
-    uint16_t last_used, last_used_chain, num;
+    uint16_t last_used;
  
      if (!vhost_svq_more_used(svq)) {
          return svq_elem;
@@ -428,11 +433,8 @@ static SVQElement vhost_svq_get_buf(VhostShadowVirtqueue 
*svq, uint32_t *len)
          return svq_elem;
      }
  
-    num = svq_elem.elem->in_num + svq_elem.elem->out_num;
-    last_used_chain = vhost_svq_last_desc_of_chain(svq, num, used_elem.id);
-    svq->desc_next[last_used_chain] = svq->free_head;
+    svq->desc_next[svq_elem.last_chain_id] = svq->free_head;
      svq->free_head = used_elem.id;
-
      *len = used_elem.len;
      return svq_elem;
  }