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:

quoted

On 2020/6/10 下午7:05, Michael S. Tsirkin wrote:

quoted

quoted

quoted

+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:

quoted

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

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