On 2020/6/11 下午5:06, Michael S. Tsirkin wrote:
On Thu, Jun 11, 2020 at 11:02:57AM +0800, Jason Wang wrote:
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
I don't think that happens. vhost_discard_avail_buf(1) is only called
after get vhost_get_avail_buf. vhost_get_avail_buf increments
first_desc. unfetch_descs only counts from first_desc to ndescs.
If I'm wrong, could you show values of first_desc and ndescs in this
scenario?
You're right, fetch_descriptor could not be called directly from the
device code.
Thanks
