On Wed, Aug 09, 2017 at 10:38:10AM +0800, Jason Wang wrote:

I think don't think current code can work well if vq.num is grater than
2^15. Since all cached idx is u16. This looks like a bug which needs to be
fixed.

That's a limitation of virtio 1.0.

quoted

* else if the interval of vq.num is [2^15, 2^16):
the logic in the original patch (809ecb9bca6a9) suffices
* else (= less than 2^15) (optional):
checking only (vring_need_event(vq->last_used_event, new + vq->num, new)
would suffice.

Am I missing something, or is this irrelevant?

Could you pls repost the suggestion copying virtio-dev mailing list
(subscriber only, sorry about that, but host/guest ABI discussions
need to copy that list)?

Looks not, I think this may work. Let me do some test.

Thanks

I think that at this point it's prudent to add a feature bit
as the virtio spec does not require to never move the event index back.