Jakub Kicinski wrote:

On Wed, 19 Aug 2020 22:22:23 +0200 Lorenzo Bianconi wrote:

quoted

quoted

On Wed, 19 Aug 2020 15:13:51 +0200 Lorenzo Bianconi wrote:  

quoted

Enable the capability to receive jumbo frames even if the interface is
running in XDP mode

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>  

Hm, already? Is all the infra in place? Or does it not imply
multi-buffer.

with this series mvneta supports xdp multi-buff on both rx and tx sides (XDP_TX
and ndo_xpd_xmit()) so we can remove MTU limitation.

Is there an API for programs to access the multi-buf frames?

Hi Lorenzo,

This is not enough to support multi-buffer in my opinion. I have the
same comment as Jakub. We need an API to pull in the multiple
buffers otherwise we break the ability to parse the packets and that
is a hard requirement to me. I don't want to lose visibility to get
jumbo frames.

At minimum we need a bpf_xdp_pull_data() to adjust pointer. In the
skmsg case we use this,

  bpf_msg_pull_data(u32 start, u32 end, u64 flags)

Where start is the offset into the packet and end is the last byte we
want to adjust start/end pointers to. This way we can walk pages if
we want and avoid having to linearize the data unless the user actual
asks us for a block that crosses a page range. Smart users then never
do a start/end that crosses a page boundary if possible. I think the
same would apply here.

XDP by default gives you the first page start/end to use freely. If
you need to parse deeper into the payload then you call bpf_msg_pull_data
with the byte offsets needed.

Also we would want performance numbers to see how good/bad this is
compared to the base case.

Thanks,
John