Re: [PATCH iwl-next 11/12] idpf: convert header split mode to libeth + napi_build_skb()

[Willem de Bruijn]

Alexander Lobakin wrote:
> Currently, idpf uses the following model for the header buffers:
> 
> * buffers are allocated via dma_alloc_coherent();
> * when receiving, napi_alloc_skb() is called and then the header is
>   copied to the newly allocated linear part.
> 
> This is far from optimal as DMA coherent zone is slow on many systems
> and memcpy() neutralizes the idea and benefits of the header split. 

In the previous revision this assertion was called out, as we have
lots of experience with the existing implementation and a previous one
based on dynamic allocation one that performed much worse. You would
share performance numbers in the next revision

https://lore.kernel.org/netdev/0b1cc400-3f58-4b9c-a08b-39104b9f2d2d@xxxxxxxxx/T/#me85d509365aba9279275e9b181248247e1f01bb0

This may be so integral to this patch series that asking to back it
out now sets back the whole effort. That is not my intent.

And I appreciate that in principle there are many potential
optizations.

But this (OOT) driver is already in use and regressions in existing
workloads is a serious headache. As is significant code churn wrt
other still OOT feature patch series.

This series (of series) modifies the driver significantly, beyond the
narrow scope of adding XDP and AF_XDP.

> Not
> speaking of that XDP can't be run on DMA coherent buffers, but at the
> same time the idea of allocating an skb to run XDP program is ill.
> Instead, use libeth to create page_pools for the header buffers, allocate
> them dynamically and then build an skb via napi_build_skb() around them
> with no memory copy. With one exception...
> When you enable header split, you except you'll always have a separate
> header buffer, so that you could reserve headroom and tailroom only
> there and then use full buffers for the data. For example, this is how
> TCP zerocopy works -- you have to have the payload aligned to PAGE_SIZE.
> The current hardware running idpf does *not* guarantee that you'll
> always have headers placed separately. For example, on my setup, even
> ICMP packets are written as one piece to the data buffers. You can't
> build a valid skb around a data buffer in this case.
> To not complicate things and not lose TCP zerocopy etc., when such thing
> happens, use the empty header buffer and pull either full frame (if it's
> short) or the Ethernet header there and build an skb around it. GRO
> layer will pull more from the data buffer later. This W/A will hopefully
> be removed one day.
> 
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>