2018-04-25 21:00 GMT+02:00 Willem de Bruijn <willemdebruijn.ker...@gmail.com>:
[...]
>>> static inline struct xdp_desc *xskq_peek_desc(struct xsk_queue *q,
>>> + struct xdp_desc *desc)
>>> +{
>>> + struct xdp_rxtx_ring *ring;
>>> +
>>> + if (q->cons_tail == q->cons_head) {
>>> + WRITE_ONCE(q->ring->consumer, q->cons_tail);
>>> + q->cons_head = q->cons_tail + xskq_nb_avail(q,
>>> RX_BATCH_SIZE);
>>> +
>>> + /* Order consumer and data */
>>> + smp_rmb();
>>> +
>>> + return xskq_validate_desc(q, desc);
>>> + }
>>> +
>>> + ring = (struct xdp_rxtx_ring *)q->ring;
>>> + *desc = ring->desc[q->cons_tail & q->ring_mask];
>>> + return desc;
>>>
>>> This only validates descriptors if taking the branch.
>>
>> Yes, that is because we only want to validate the descriptors once
>> even if we call this function multiple times for the same entry.
>
> Then I am probably misreading this function. But isn't head increased
> by up to RX_BATCH_SIZE frames at once. If so, then for many frames
> the branch is not taken.
You're not misreading it! :-) The head is indeed increased, but only
the tail descriptor is validated in that function. Later in the
xskq_discard_desc function when the tail is moved, the next descriptor
is validated. So, the peek function will always return a validated
descriptor, but the validation can be done in either peek or discard.
Björn
