Re: [Qemu-devel] [PATCH v4 09/11] virtio-net: update the head descriptor in a chain lastly

[Wei Xu <wexu@redhat.com>]

On Tue, Feb 19, 2019 at 03:23:01PM +0800, Jason Wang wrote:
> 
> On 2019/2/14 下午12:26, wexu@redhat.com wrote:
> >From: Wei Xu <wexu@redhat.com>
> >
> >This is a helper for packed ring.
> >
> >To support packed ring, the head descriptor in a chain should be updated
> >lastly since no 'avail_idx' like in packed ring to explicitly tell the
> >driver side that all payload is ready after having done the chain, so
> >the head is always visible immediately.
> >
> >This patch fills the header after done all the other ones.
> >
> >Signed-off-by: Wei Xu <wexu@redhat.com>
> 
> 
> It's really odd to workaround API issue in the implementation of device.
> Please introduce batched used updating helpers instead.
Can you elaborate a bit more? I don't get it as well.

The exact batch as vhost-net or dpdk pmd is not supported by userspace
backend. The change here is to keep the header descriptor updated at
last in case of a chaining descriptors and the helper might not help
too much.

Wei
> 
> Thanks
> 
> 
> >---
> >  hw/net/virtio-net.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> >diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> >index 3f319ef..330abea 100644
> >--- a/hw/net/virtio-net.c
> >+++ b/hw/net/virtio-net.c
> >@@ -1251,6 +1251,8 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
> >      struct virtio_net_hdr_mrg_rxbuf mhdr;
> >      unsigned mhdr_cnt = 0;
> >      size_t offset, i, guest_offset;
> >+    VirtQueueElement head;
> >+    int head_len = 0;
> >      if (!virtio_net_can_receive(nc)) {
> >          return -1;
> >@@ -1328,7 +1330,13 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
> >          }
> >          /* signal other side */
> >-        virtqueue_fill(q->rx_vq, elem, total, i++);
> >+        if (i == 0) {
> >+            head_len = total;
> >+            head = *elem;
> >+        } else {
> >+            virtqueue_fill(q->rx_vq, elem, len, i);
> >+        }
> >+        i++;
> >          g_free(elem);
> >      }
> >@@ -1339,6 +1347,7 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
> >                       &mhdr.num_buffers, sizeof mhdr.num_buffers);
> >      }
> >+    virtqueue_fill(q->rx_vq, &head, head_len, 0);
> >      virtqueue_flush(q->rx_vq, i);
> >      virtio_notify(vdev, q->rx_vq);
>