Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring

[Jason Wang <jasowang@redhat.com>]

On 2018年05月03日 21:54, Tiwei Bie wrote:
> On Thu, May 03, 2018 at 03:25:29PM +0800, Jason Wang wrote:
>> On 2018年05月03日 10:09, Tiwei Bie wrote:
>>>>>> So how about we use the straightforward way then?
>>>>> You mean we do new += vq->vring_packed.num instead
>>>>> of event_idx -= vq->vring_packed.num before calling
>>>>> vring_need_event()?
>>>>>
>>>>> The problem is that, the second param (new_idx) of
>>>>> vring_need_event() will be used for:
>>>>>
>>>>> (__u16)(new_idx - event_idx - 1)
>>>>> (__u16)(new_idx - old)
>>>>>
>>>>> So if we change new, we will need to change old too.
>>>> I think that since we have a branch there anyway,
>>>> we are better off just special-casing if (wrap_counter != vq->wrap_counter).
>>>> Treat is differenty and avoid casts.
>>>>
>>>>> And that would be an ugly hack..
>>>>>
>>>>> Best regards,
>>>>> Tiwei Bie
>>>> I consider casts and huge numbers with two's complement
>>>> games even uglier.
>>> The dependency on two's complement game is introduced
>>> since the split ring.
>>>
>>> In packed ring, old is calculated via:
>>>
>>> old = vq->next_avail_idx - vq->num_added;
>>>
>>> In split ring, old is calculated via:
>>>
>>> old = vq->avail_idx_shadow - vq->num_added;
>>>
>>> In both cases, when vq->num_added is bigger, old will
>>> be a big number.
>>>
>>> Best regards,
>>> Tiwei Bie
>>>
>> How about just do something like vhost:
>>
>> static u16 vhost_idx_diff(struct vhost_virtqueue *vq, u16 old, u16 new)
>> {
>>      if (new > old)
>>          return new - old;
>>      return  (new + vq->num - old);
>> }
>>
>> static bool vhost_vring_packed_need_event(struct vhost_virtqueue *vq,
>>                        __u16 event_off, __u16 new,
>>                        __u16 old)
>> {
>>      return (__u16)(vhost_idx_diff(vq, new, event_off) - 1) <
>>             (__u16)vhost_idx_diff(vq, new, old);
>> }
>>
>> ?
> It seems that there is a typo in above code. The second
> param of vhost_idx_diff() is `old`, but when calling this
> function in vhost_vring_packed_need_event(), `new` is
> passed as the second param.

Right.

>
> If we assume the second param of vhost_idx_diff() is new
> and the third one is old, i.e.:
>
> static u16 vhost_idx_diff(struct vhost_virtqueue *vq, u16 new, u16 old)
> {
>      if (new > old)
>          return new - old;
>      return  (new + vq->num - old);
> }
>
> I think it's still not right.
>
> Because in virtqueue_enable_cb_delayed(), we may set an
> event_off which is bigger than new and both of them have
> wrapped. And in this case, although new is smaller than
> event_off (i.e. the third param -- old), new shouldn't
> add vq->num, and actually we are expecting a very big
> idx diff.

Yes, so to calculate distance correctly between event and new, we just 
need to compare the warp counter and return false if it doesn't match 
without the need to try to add vq.num here.

Thanks

>
> Best regards,
> Tiwei Bie