[PATCH v4 2/8] vdpa: Use iovec for vhost_vdpa_net_cvq_add()

[Hawkins Jiawei <yin31149@gmail.com>]

Next patches in this series will no longer perform an
immediate poll and check of the device's used buffers
for each CVQ state load command. Consequently, there
will be multiple pending buffers in the shadow VirtQueue,
making it a must for every control command to have its
own buffer.

To achieve this, this patch refactor vhost_vdpa_net_cvq_add()
to accept `struct iovec`, which eliminates the coupling of
control commands to `s->cvq_cmd_out_buffer` and `s->status`,
allowing them to use their own buffer.

Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
---
v4:
  - split `in` to `vdpa_in` and `model_in` instead of reusing `in`
in vhost_vdpa_net_handle_ctrl_avail() suggested by Eugenio

v3: https://lore.kernel.org/all/b1d473772ec4bcb254ab0d12430c9b1efe758606.1689748694.git.yin31149@gmail.com/

 net/vhost-vdpa.c | 39 ++++++++++++++++++++++-----------------
 1 file changed, 22 insertions(+), 17 deletions(-)

diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
index 3acda8591a..a875767ee9 100644
--- a/net/vhost-vdpa.c
+++ b/net/vhost-vdpa.c
@@ -596,22 +596,14 @@ static void vhost_vdpa_net_cvq_stop(NetClientState *nc)
     vhost_vdpa_net_client_stop(nc);
 }
 
-static ssize_t vhost_vdpa_net_cvq_add(VhostVDPAState *s, size_t out_len,
-                                      size_t in_len)
+static ssize_t vhost_vdpa_net_cvq_add(VhostVDPAState *s,
+                                    const struct iovec *out_sg, size_t out_num,
+                                    const struct iovec *in_sg, size_t in_num)
 {
-    /* Buffers for the device */
-    const struct iovec out = {
-        .iov_base = s->cvq_cmd_out_buffer,
-        .iov_len = out_len,
-    };
-    const struct iovec in = {
-        .iov_base = s->status,
-        .iov_len = sizeof(virtio_net_ctrl_ack),
-    };
     VhostShadowVirtqueue *svq = g_ptr_array_index(s->vhost_vdpa.shadow_vqs, 0);
     int r;
 
-    r = vhost_svq_add(svq, &out, 1, &in, 1, NULL);
+    r = vhost_svq_add(svq, out_sg, out_num, in_sg, in_num, NULL);
     if (unlikely(r != 0)) {
         if (unlikely(r == -ENOSPC)) {
             qemu_log_mask(LOG_GUEST_ERROR, "%s: No space on device queue\n",
@@ -637,6 +629,15 @@ static ssize_t vhost_vdpa_net_load_cmd(VhostVDPAState *s, uint8_t class,
         .cmd = cmd,
     };
     size_t data_size = iov_size(data_sg, data_num);
+    /* Buffers for the device */
+    const struct iovec out = {
+        .iov_base = s->cvq_cmd_out_buffer,
+        .iov_len = sizeof(ctrl) + data_size,
+    };
+    const struct iovec in = {
+        .iov_base = s->status,
+        .iov_len = sizeof(*s->status),
+    };
 
     assert(data_size < vhost_vdpa_net_cvq_cmd_page_len() - sizeof(ctrl));
 
@@ -647,8 +648,7 @@ static ssize_t vhost_vdpa_net_load_cmd(VhostVDPAState *s, uint8_t class,
     iov_to_buf(data_sg, data_num, 0,
                s->cvq_cmd_out_buffer + sizeof(ctrl), data_size);
 
-    return vhost_vdpa_net_cvq_add(s, data_size + sizeof(ctrl),
-                                  sizeof(virtio_net_ctrl_ack));
+    return vhost_vdpa_net_cvq_add(s, &out, 1, &in, 1);
 }
 
 static int vhost_vdpa_net_load_mac(VhostVDPAState *s, const VirtIONet *n)
@@ -1222,10 +1222,15 @@ static int vhost_vdpa_net_handle_ctrl_avail(VhostShadowVirtqueue *svq,
         .iov_base = s->cvq_cmd_out_buffer,
     };
     /* in buffer used for device model */
-    const struct iovec in = {
+    const struct iovec model_in = {
         .iov_base = &status,
         .iov_len = sizeof(status),
     };
+    /* in buffer used for vdpa device */
+    const struct iovec vdpa_in = {
+        .iov_base = s->status,
+        .iov_len = sizeof(*s->status),
+    };
     ssize_t dev_written = -EINVAL;
 
     out.iov_len = iov_to_buf(elem->out_sg, elem->out_num, 0,
@@ -1259,7 +1264,7 @@ static int vhost_vdpa_net_handle_ctrl_avail(VhostShadowVirtqueue *svq,
             goto out;
         }
     } else {
-        dev_written = vhost_vdpa_net_cvq_add(s, out.iov_len, sizeof(status));
+        dev_written = vhost_vdpa_net_cvq_add(s, &out, 1, &vdpa_in, 1);
         if (unlikely(dev_written < 0)) {
             goto out;
         }
@@ -1275,7 +1280,7 @@ static int vhost_vdpa_net_handle_ctrl_avail(VhostShadowVirtqueue *svq,
     }
 
     status = VIRTIO_NET_ERR;
-    virtio_net_handle_ctrl_iov(svq->vdev, &in, 1, &out, 1);
+    virtio_net_handle_ctrl_iov(svq->vdev, &model_in, 1, &out, 1);
     if (status != VIRTIO_NET_OK) {
         error_report("Bad CVQ processing in model");
     }
-- 
2.25.1