Messages in this thread | | | Date | Thu, 3 May 2018 21:54:30 +0800 | From | Tiwei Bie <> | Subject | Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring |
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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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