Re: AF_XDP RX processing with NO rx-ring

On Thu, 5 Oct 2023 at 16:31, Srivats P <[email protected]> wrote:
>
> Hi,
>
> I want to clarify my AF_XDP understanding for a particular scenario.
>
> Consider a single UMEM with 1 Fill Ring and 1 Completion Ring and
> single XDP socket  bound to queue 0 with 1 Tx Ring but NO Rx Ring.
> Assume the NIC has only a single queue (to keep things simple for
> explaining this scenario).
>
> There is a XDP program attached to Queue 0 which does either a
> XDP_DROP or XDP_PASS for all Rx packets.
>
> We are running in Driver mode.
>
> What happens to RX packets received on Queue 0?
>
> Here's my understanding -
>
>  * Even though there is no AF_XDP Rx Ring, there will be a NIC Rx Ring
> for queue 0
>  * The NIC Rx Ring for queue 0 is populated by the driver with UMEM
> buffers taken from the Fill ring

In driver mode (i.e. not zero-copy mode) this will not happen. The
ring will be populated by kernel buffers.

>  * A Rx packet will be received in the NIC Rx Ring for Queue 0 first
>  * The driver will run the XDP program on the Rx packet buffer (a UMEM buffer)
>  * If the program results in XDP_DROP, the driver will "free" the Umem
> buffer by putting it on the Completion Ring
>  * If the program results in XDP_PASS, the driver will allocate a
> standard Linux kernel SKB, copy the packet buffer contents into the
> SKB and queue it up for standard netdev processing; It will then
> "free" the RX Umem buffer by putting it on the Completion Ring (since
> we have already copied packet into the skb)

As the buffers are kernel buffers, user-space will not be notified.
The completion ring is solely for the Tx path, saying that user space
can have the buffer back.

The rest is correct.

/Magnus

> Is this understanding correct or am I mistaken anywhere?
>
> Thanks in advance,
> Srivats