Re: [Qemu-devel] Question about libvhost-user and vhost-user-bridge.c

[Stefan Hajnoczi]

On Wed, Aug 14, 2019 at 10:54:34AM -0700, William Tu wrote:
> Hi,
> 
> I'm using libvhost-user.a to write a vhost backend, in order to receive and
> send packets from/to VMs from OVS. I started by reading the 
> vhost-user-bridge.c.
> I can now pass the initialization stage, seeing .queue_set_started get 
> invoked.
> 
> However, I am stuck at receiving the packet from VM.
> So is it correct to do:
> 1) check vu_queue_empty, started, and aval_bytes, if OK, then

This step can be skipped because vu_queue_pop() returns NULL if there
are no virtqueue elements available.

> 2) elem = vu_queue_pop(&dev->vudev, vq, sizeof(VuVirtqElement));
> 3) the packet payload should be at elem->in_sg->iov_base + hdrlen? or
> at elem->out_sg?

The driver->device buffers are elem->out_sg and the device->driver
buffers are elem->in_sg.

Device implementations must not make assumptions about the layout of
out_sg and in_sg (e.g. you cannot assume that in_sg[0]->iov_len ==
sizeof(struct virtio_net_hdr) and you must handle the case where
in_sg[0]->iov_len == 1).

> I tried to hex dump the iov_base, but the content doesn't look like
> having a ethernet header. I saw in vubr_backend_recv_cb at 
> vhost-user-bridge.c,
> we're creating another iovec and recvmsg(vubr->backend_udp_sock, &msg, 0);
> I don't think I have to create backend UDP sock, am I correct?

Please see the VIRTIO specification for details of the virtio-net rx/tx
virtqueue formats:
https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-2050006

I think you may need to handle the struct virtio_net_hdr that comes
before the Ethernet header.

Stefan

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