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

[William Tu <u9012063@gmail.com>]

Hi Stefan,

Thanks for your reply.

On Thu, Aug 15, 2019 at 7:07 AM Stefan Hajnoczi <stefanha@gmail.com> wrote:
>
> 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.

OK, thanks. Then for vswitch to receive from qemu, I should check
device->driver.
>
> 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).

OK so I might need to copy to a single continuous buffer.

>
> > 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.

Thank, will look at it.
William
>
> Stefan