Re: [Qemu-devel] [virtio guest] vring_need_event() from virtqueue_kick_prepare()

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Jul 07, 2015 at 02:43:06PM +0300, Catalin Vasile wrote:
> My vhost module respects the format vhost-net uses:
> 
> /* <code> summary */
> mutex_lock(&vq->mutex);
> vhost_disable_notify();
> for (;;) {
>         head = vhost_get_vq_desc();
>        if (head == vq->num) {
>             if (unlikely(vhost_enable_notify())) {
>                 vhost_disable_notify();
>                 continue;
>             }
>             break;
>        }
>        vhost_add_used_and_signal();
> }
> mutex_unlock(&vq->mutex);
> /* </code> */
> 
> I have made a lot of printk() calls and the first job gets processed
> completely, and gets through all those calls:
> 1. it goes into a first loop and processes the first job (get
> descriptor, work with the descriptor, add used and signal).
> 2. On the second loop it hits head == vq->num, and goes back to
> listening to notifications (successfully, it does not get into the
> fallback).
> 
> Now in the guest:
> 1. sends first job and the paramers used to call vring_need_event() are:
> vring_avail_event=0, new=1, old=0 (which makes the function evaluate to "0 < 1")
> 2. the queue is kicked and vhost does its job.
> 3. the guest driver reaches the end of the first job, and lets the
> following job take its course, only this time vring_need_event()
> receives the following parameters:
> vring_avail_event=0, new=2, old=1 (which makes the function evaluate to "1 < 1")

This means you need to look at the vhost code because vring_avail_event
shouldn't be 0.

Why wasn't vhost_update_avail_event() called?

Maybe the feature bit negotiation for your device is broken and vhost
thinks VIRTIO_RING_F_EVENT_IDX is not set.

Or does the used ring have the VRING_USED_F_NO_NOTIFY flag?

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