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

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Date:   Tue, 8 May 2018 17:16:28 +0800
From:   Tiwei Bie <tiwei.bie@...el.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        wexu@...hat.com, jfreimann@...hat.com
Subject: Re: [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`.

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
> > > 
>