Re: [Qemu-devel] [PATCH] vhost: don't set vring call if no vector

[Jason Wang <jasowang@redhat.com>]

On 2016年08月02日 14:37, Cornelia Huck wrote:
> On Tue, 2 Aug 2016 10:39:22 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
>> On 2016年08月02日 02:00, Cornelia Huck wrote:
>>> On Mon,  1 Aug 2016 16:07:58 +0800
>>> Jason Wang <jasowang@redhat.com> wrote:
>>>> +    if (k->query_guest_notifiers &&
>>>> +        k->query_guest_notifiers(qbus->parent) &&
>>>> +        virtio_queue_vector(vdev, idx) == VIRTIO_NO_VECTOR) {
>>> I'm trying to imagine what this means for virtio-ccw. Keep in mind that
>>> we don't have the concept of setting a 'vector' by the OS (the vector
>>> is setup internally to the queue index and the OS does not see it.)
>>>
>>> ->query_guest_notifiers() is true if the OS has enabled the subchannel
>>> of the proxy device (i.e., if it is enabled for doing *anything* with
>>> the subchannel, regardless whether the OS wants to be notified or is
>>> planning to poll.) The second condition will never hold true for any
>>> valid queue once the OS has setup the queues.
>> I see, so If I understand correctly, there's no way to detect whether or
>> not guest will use a specific virtqueue interrupt?
> Yes. The guest will either be notified for any virtqueue (if it
> registered indicators; this is always done for every vq of the device
> at once), or for none.
>
>>> So this won't break anything for virtio-ccw AFAICS, but I don't think
>>> we gain anything.
>> Yes, but if we could infer whether or not polling is used in the driver,
>> this is probably all we can do for ccw.
> What we could do is check whether the driver has registed indicators
> and disable setting up notification for any vq of the device if not.
> But I'm not sure an always-polling driver is worth optimizing for.

It's worth for at least pci transport. Consider virito-net pmd in guest, 
a NULL vring call can save unnecessary userspace memory access and 
memory barriers.