Linux Virtualization - May 2018 - [RFC v3 4/5] virtio_ring: add event idx support in packed ring

On Tue, May 08, 2018 at 03:16:53PM +0800, Jason Wang
wrote:
> On 2018?05?08? 14:44, Tiwei Bie wrote:
> > On Tue, May 08, 2018 at 01:40:40PM +0800, Jason Wang wrote:
> > > On 2018?05?08? 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`.

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
> > > 
>

On 2018?05?08? 17:16, Tiwei Bie wrote:
> On Tue, May 08, 2018 at 03:16:53PM +0800, Jason Wang wrote:
>> On 2018?05?08? 14:44, Tiwei Bie wrote:
>>> On Tue, May 08, 2018 at 01:40:40PM +0800, Jason Wang wrote:
>>>> On 2018?05?08? 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
>>>>

On Tue, May 08, 2018 at 05:34:40PM +0800, Jason Wang
wrote:
> On 2018?05?08? 17:16, Tiwei Bie wrote:
> > On Tue, May 08, 2018 at 03:16:53PM +0800, Jason Wang wrote:
> > > On 2018?05?08? 14:44, Tiwei Bie wrote:
> > > > On Tue, May 08, 2018 at 01:40:40PM +0800, Jason Wang wrote:
> > > > > On 2018?05?08? 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).
If vhost is faster than virtio driver, I guess above
case may happen. The `old` and `new` will be updated
each time we want to notify the driver. If the driver
is slower, `old` and `new` in vhost may wrap before
the `off` which is set by driver wraps.

Best regards,
Tiwei Bie
> 
> 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
> > > > > 
>