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);
}
?
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