marvell/octeontx2: XDP vs skb headroom reservation

Hi,

I'm auditing calls to skb_metadata_set() in drivers and something seems off in oceontx2. I may be completely misreading the code. I'm a total stranger to that driver.

It would seem that there is a difference between the amount of headroom that the driver reserves for the XDP buffer vs the skb that is built from on that buffer, while the two should match.

In the receive handler we have:

otx2_rcv_pkt_handler
otx2_xdp_rcv_pkt_handler if pfvf->xdp_prog

    xdp_prepare_buff(..., OTX2_HEAD_ROOM, ...)
      where OTX2_HEAD_ROOM = OTX2_ALIGN = 128

skb = napi_get_frags
napi_alloc_skb
skb_reserve(NET_SKB_PAD + NET_IP_ALIGN)
where NET_SKB_PAD = 64, NET_IP_ALIGN = 0 on x86
napi_gro_frags

There are no other calls to skb_reserve there, so we seem to have a mismatch. 128B of headroom in XDP buff, and only 64B of headroom in skb head.

Again, I could be totally wrong about this, I'm not familiar with the driver, but I thought you might want to double check this.

Thanks,
-jkbs

Hi,

The otx2 netdev driver calculates the receive buffer size to accommodate the full network packet as follows:
hw.buf_len = mtu + OTX2_ETH_HLEN + OTX2_HW_TIMESTAMP_LEN;
In addition, it reserves OTX2_HEAD_ROOM (128 bytes). The hardware actually copies
the packet starting at (u64*)bufptr + OTX2_HEAD_ROOM. This means the SKB has
128 bytes of headroom available, which is especially useful in the XDP case for adding any extra headers.

static int otx2_get_rbuf_size(struct otx2_nic *pf, int mtu)

{
        int frame_size;
        int total_size;
        int rbuf_size;

        if (pf->hw.rbuf_len)
                return ALIGN(pf->hw.rbuf_len, OTX2_ALIGN) + OTX2_HEAD_ROOM;

Thanks,
Geetha.