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Subject: Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring
To:     Tiwei Bie <tiwei.bie@intel.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        wexu@redhat.com, jfreimann@redhat.com
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <122277c6-d103-e1f6-d695-4d64e6934a51@redhat.com>
Date:   Tue, 8 May 2018 17:34:40 +0800
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On 2018年05月08日 17:16, Tiwei Bie wrote:
> On Tue, May 08, 2018 at 03:16:53PM +0800, Jason Wang wrote:
>> On 2018年05月08日 14:44, Tiwei Bie wrote:
>>> On Tue, May 08, 2018 at 01:40:40PM +0800, Jason Wang wrote:
>>>> On 2018年05月08日 11:05, Jason Wang wrote:
>>>>>> 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
>>>> Sorry, looks like the following should work, we need add vq.num if
>>>> used_wrap_counter does not match:
>>>>
>>>> static bool vhost_vring_packed_need_event(struct vhost_virtqueue *vq,
>>>>                         __u16 off_wrap, __u16 new,
>>>>                         __u16 old)
>>>> {
>>>>       bool wrap = off_wrap >> 15;
>>>>       int off = off_wrap & ~(1 << 15);
>>>>       __u16 d1, d2;
>>>>
>>>>       if (wrap != vq->used_wrap_counter)
>>>>           d1 = new + vq->num - off - 1;
>>> Just to draw your attention (maybe you have already
>>> noticed this).
>> I miss this, thanks!
>>
>>> In this case (i.e. wrap != vq->used_wrap_counter),
>>> it's also possible that (off < new) is true. Because,
>>>
>>> when virtqueue_enable_cb_delayed_packed() is used,
>>> `off` is calculated in driver in a way like this:
>>>
>>> 	off = vq->last_used_idx + bufs;
>>> 	if (off >= vq->vring_packed.num) {
>>> 		off -= vq->vring_packed.num;
>>> 		wrap_counter ^= 1;
>>> 	}
>>>
>>> And when `new` (in vhost) is close to vq->num. The
>>> vq->last_used_idx + bufs (in driver) can be bigger
>>> than vq->vring_packed.num, and:
>>>
>>> 1. `off` will wrap;
>>> 2. wrap counters won't match;
>>> 3. off < new;
>>>
>>> And d1 (i.e. new + vq->num - off - 1) will be a value
>>> bigger than vq->num. I'm okay with this, although it's
>>> a bit weird.
>>
>> So I'm considering something more compact by reusing vring_need_event() by
>> pretending a larger queue size and adding vq->num back when necessary:
>>
>> static bool vhost_vring_packed_need_event(struct vhost_virtqueue *vq,
>>                        __u16 off_wrap, __u16 new,
>>                        __u16 old)
>> {
>>      bool wrap = vq->used_wrap_counter;
> If the wrap counter is obtained from the vq,
> I think `new` should also be obtained from
> the vq. Or the wrap counter should be carried
> in `new`.
>
>>      int off = off_wrap & ~(1 << 15);
>>      __u16 d1, d2;
>>
>>      if (new < old) {
>>          new += vq->num;
>>          wrap ^= 1;
>>      }
>>
>>      if (wrap != off_wrap >> 15)
>>          off += vq->num;
> When `new` and `old` wraps, and `off` doesn't wrap,
> wrap != (off_wrap >> 15) will be true. In this case,
> `off` is bigger than `new`, and what we should do
> is `off -= vq->num` instead of `off += vq->num`.

If I understand this correctly, if we track old correctly, it won't 
happen if guest driver behave correctly. That means it should only 
happen for a buggy driver (e.g trying to move off_wrap back).

Thanks

>
> Best regards,
> Tiwei Bie
>
>>      return vring_need_event(off, new, old);
>> }
>>
>>
>>> Best regards,
>>> Tiwei Bie
>>>
>>>>       else
>>>>           d1 = new - off - 1;
>>>>
>>>>       if (new > old)
>>>>           d2 = new - old;
>>>>       else
>>>>           d2 = new + vq->num - old;
>>>>
>>>>       return d1 < d2;
>>>> }
>>>>
>>>> Thanks
>>>>