On Thu, Oct 5, 2023 at 9:03 PM Magnus Karlsson <magnus.karlsson@xxxxxxxxx> wrote: > > On Thu, 5 Oct 2023 at 17:18, Srivats P <pstavirs@xxxxxxxxx> wrote: > > > > On Thu, Oct 5, 2023 at 8:40 PM Magnus Karlsson > > <magnus.karlsson@xxxxxxxxx> wrote: > > > > > > 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. > > > > What is the behaviour in zero-copy mode? > > Then the behaviour is according to what you wrote above. Just note > that nothing will be returned in the fill ring as you are not sending > anything to the AF_XDP socket. I assume you meant "Rx Ring" and not "Fill ring" above? > > > > > > > > * 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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