On Thu, 5 Oct 2023 at 16:31, Srivats P <pstavirs@xxxxxxxxx> 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
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