[Lguest] Possible bug? adding only one buff to a vq with one input queue

[Earlence Fernandes]

This is the code for my probe function:

static int virtsyscalldev_probe(struct virtio_device *vdev)
{
struct virtio_syscalldev *vsdd;
 struct virtqueue *vqs[1];
struct scatterlist sg[1];
int err, nvqs;
 vq_callback_t *callbacks[] = { fromhost_dosyscall };
const char *names[] = { "dosyscall_q" };
 vdev->priv = vsdd = kmalloc(sizeof(*vsdd), GFP_KERNEL);
if(!vsdd)
 {
err = -ENOMEM;
goto out;
}
 vsdd->vdev = vdev; //cache the virtio_device
err = vdev->config->find_vqs(vdev, nvqs, vqs, callbacks, names);
 if(err)
goto out_free_vsdd;
 vsdd->input_q = vqs[0];
 /*
prime the input_q with a buffer so the hypervisor (host) can signal
 us by placing data into it and inducing an interrupt
*/
sg_init_one(sg, vsdd->inbuff, sizeof(vsdd->inbuff));
 printk(KERN_INFO "buffer ready to prime3\n");
if(virtqueue_add_buf(vsdd->input_q, sg, 0, 1, vsdd) < 0)
 {
printk(KERN_ERR "OOOOOOOOOOOOPS\n");
BUG();
 }
 //buffer has been loaded in the vq, so now kick it
 //and hence notify the hypervisor (host)
virtqueue_kick(vsdd->input_q);
 return 0;
out_free_vsdd:
kfree(vsdd);
out:
return err;
}

The syscalldev struct has a char buffer.
In the launcher, I created a single virtqueue like so:

add_virtqueue(dev, VIRTQUEUE_NUM, syscall_toguest);

I still get the BUG message. Any idea why?

-Earlence


On Sat, Jun 8, 2013 at 11:38 PM, Earlence Fernandes <earlenceferns at gmail.com
> wrote:

> I am trying to achieve only host->guest communication and so I've created
> a virtio driver for the guest kernel which calls virtio_add_buf_gfp with
> one inbuf and 0 outbufs.
>
> Whenever I boot the system, lguest crashes complaining of a kernel BUG. At
> the line specified in virtio_ring.c, I see the following:
>
> BUG_ON(out + in > vq->vring.num);
>
> From this, it seems to me that I need more vrings than buffers.
>
> So, how do I have a channel from the host launcher to the guest kernel
> with only one queue which is used to send data exclusively into the guest?
>
> I've been following virtio_balloon as an example.
>
> -Earlence
>
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