Subject: Re: [PATCH bpf-next V2 0/7] xdp: Allow BPF to set RX hints for XDP_REDIRECTed packets

From: Jakub Kicinski <[email-masked]>
Date: Thu, 17 Jul 2025 18:25:34 -0700

On Thu, 17 Jul 2025 15:08:49 +0200 Jesper Dangaard Brouer wrote:
> Let me explain why it is a bad idea of writing into the RX descriptors.
> The DMA descriptors are allocated as coherent DMA (dma_alloc_coherent).
> This is memory that is shared with the NIC hardware device, which
> implies cache-line coherence.  NIC performance is tightly coupled to
> limiting cache misses for descriptors.  One common trick is to pack more
> descriptors into a single cache-line.  Thus, if we start to write into
> the current RX-descriptor, then we invalidate that cache-line seen from
> the device, and next RX-descriptor (from this cache-line) will be in an
> unfortunate coherent state.  Behind the scene this might lead to some
> extra PCIe transactions.
> 
> By writing to the xdp_frame, we don't have to modify the DMA descriptors
> directly and risk invalidating cache lines for the NIC.

I thought you main use case is redirected packets. In which case it's
the _remote_ end that's writing its metadata, if it's veth it's
obviously not going to be doing it into DMA coherent memory.

The metadata travels between the source and destination in program-
-defined format.

> Thanks for the feedback. I can see why you'd be concerned about adding
> another adhoc scheme or making xdp_frame grow into a "para-skb".
> 
> However, I'd like to frame this as part of a long-term plan we've been
> calling the "mini-SKB" concept. This isn't a new idea, but a
> continuation of architectural discussions from as far back as [2016].

My understanding is that while this was floated as a plan by some,
nobody came up with a clean way of implementing it.