Re: [Qemu-devel] virtio-net with virtio-mmio

[Ying-Shiuan Pan]

Hi, Stefan

Thanks for your response.


2011/12/30 Stefan Hajnoczi <address@hidden>
>
> On Wed, Dec 28, 2011 at 06:16:42PM +0800, Ying-Shiuan Pan wrote:
> > I'm very interested in virtio-mmio Peter Maydell did for QEMU,
> > (http://lists.nongnu.org/archive/html/qemu-devel/2011-11/msg01870.html)
> >
> > actually, I've tested the virtio-blk, and it is working.
> > I applied those patch to QEMU-1.0 and brought the virtio_mmio.c from
> > Linux-3.2-rc back to Linux-linaro-2.6.38.
> >
> > I also found a small bug in virito-mmio.c: virtio_mmio_write(),
> > Peter forgot to break in each case of switch block.
> > After fixed the small bug, the virtio-balloon works as well.
>
> PMM: Do you have a git branch where you could apply Ying-Shiuan's fix?
hmm... I've sent Peter my patch :)
>
> > Then, when I attempted to enable the virtio-net, the initialization part is
> > fine,
> > however, the QEMU crashed and printed this message:
> > "virtio-net header not in first element"
> >
> > It happens when the front-end virtio-net is invoking virtqueue_kick() at
> > the end of try_fill_recv().
> > Then, QEMU gets this message and invokes virtio_net_receive(), then the
> > error happens.
>
> virtio-net is looking at a virtqueue element (a packet buffer provided
> by the guest OS into which QEMU will copy the received packet) but the
> virtqueue element does not have the expected format.  You can check the
> virtio specification for the details on the virtio-net buffer layout:
> http://ozlabs.org/~rusty/virtio-spec/
>
> It sounds like the vring is corrupted or QEMU's virtio code is not
> correctly reading the virtqueue element (which is basically an iovec
> array).
>
> virtio-net is more advanced than virtio-blk in how it uses virtio so
> it's not surprising that you've discovered an issue.  Debugging this
> comes down to looking at the vring descriptors and the virtqueue
> elements that QEMU extracts.
>
> It sounds like there's a problem with the rx queue, you could try
> indirect_desc=off to disable indirect vring descriptors to see if that
> is part of the problem.
>
> Stefan
I guessed that the problem was in adding header,
virtio-net should insert its header to the ring buffer before its
packet, correct me if I'm wrong.

virtio-net driver (backend part) can use add_recvbuf_small() /
add_recvbuf_big() / add_recvbuf_mergeable()
depending on what features the virtio-net device (QEMU part) provides.
I found that both small and big add the header, while the mergeable
one does not.
Besides, in that add_recvbuf_big(), the comments mentioned about a QEMU bug??

Anyway, I found a workaround solution, that is disable the
VIRTIO_NET_F_MRG_RXBUF feature
and make virtio-net driver use add_recvbuf_big()

However, I am still not clear about how the problem happened.
If I luckily figure out, I'll post it. :)

So~ currently, virito-balloon, virtio-blk, and virtio-net can work
properly  with virtio-mmio. :D

The following is the *workaround solution* which is for virtio-net
driver part (frontend).

diff --git a/virtio_net.c b/virtio_net.c
index 82dba5a..811e2d9 100644
--- a/virtio_net.c
+++ b/virtio_net.c
@@ -955,8 +955,10 @@ static int virtnet_probe(struct virtio_device *vdev)
            virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_ECN))
                vi->big_packets = true;

+#ifndef ARM
        if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
                vi->mergeable_rx_bufs = true;
+#endif

        /* We expect two virtqueues, receive then send,
         * and optionally control. */


----
Best Regards,
Ying-Shiuan Pan