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.
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.
Best regards,
Tiwei Bie
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