Messages in this thread | | | Subject | Re: [PATCH RFC v6 02/11] vhost: use batched get_vq_desc version | From | Jason Wang <> | Date | Thu, 11 Jun 2020 11:02:57 +0800 |
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On 2020/6/10 下午7:05, Michael S. Tsirkin wrote: >>> +EXPORT_SYMBOL_GPL(vhost_get_vq_desc); >>> /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */ >>> void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n) >>> { >>> + unfetch_descs(vq); >>> vq->last_avail_idx -= n; >> So unfetch_descs() has decreased last_avail_idx. >> Can we fix this by letting unfetch_descs() return the number and then we can >> do: >> >> int d = unfetch_descs(vq); >> vq->last_avail_idx -= (n > d) ? n - d: 0; >> >> Thanks > That's intentional I think - we need both.
Yes, but:
> > Unfetch_descs drops the descriptors in the cache that were > *not returned to caller* through get_vq_desc. > > vhost_discard_vq_desc drops the ones that were returned through get_vq_desc. > > Did I miss anything?
We could count some descriptors twice, consider the case e.g we only cache on descriptor:
fetch_descs() fetch_buf() last_avail_idx++;
Then we want do discard it: vhost_discard_avail_buf(1) unfetch_descs() last_avail_idx--; last_avail_idx -= 1;
Thanks
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