netdev - Re: [PATCH bpf-next 13/15] xsk: support for Tx

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Date:   Thu, 26 Apr 2018 06:02:24 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Magnus Karlsson <magnus.karlsson@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        michael.lundkvist@...csson.com,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Singhai, Anjali" <anjali.singhai@...el.com>,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>
Subject: Re: [PATCH bpf-next 13/15] xsk: support for Tx

2018-04-25 21:00 GMT+02:00 Willem de Bruijn <willemdebruijn.kernel@...il.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