Re: Virtio Spec clarification

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 31, 2017 at 01:13:37PM -0500, Karandeep Chahal wrote:
> Hello Virtio Developers,
> 
> I need some clarification about the virtio spec, and I was wondering if you
> can help me with that.
> 
> If VIRTIO_F_EVENT_IDX feature is negotiated, when is the driver supposed to
> update the used_event? Is the driver supposed to do it before or after it
> updates the avail_idx?

user_event is normally unrelated to available index.

used_event is related to processing used ring entries.
It requests an interrupt each time the used index will reach
a specific value.
After updating it, the spec suggests that the driver check that the
index did not reach that value yet.


> The reason I am asking this question is because if the driver updates the
> used_event after it updates the avail_idx, there is a possibility that the
> device will process and complete the IO before the driver can update the
> used_event. My device is constantly polling for changes to avail_idx, and
> sometimes it does not interrupt the driver because the used_event has not
> been updated by the driver yet.
> 
> Thanking you in anticipation.
> 
> Sincerely,
> K. Chahal
> 
> P.S. Sorry I had to resend this email because it was not delivered the first
> time around. I was not a subscriber to the mailing list.

I think this should not be a problem if driver checks the
used index after updating used_event.




-- 
MST