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

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 11 Jul 2025 09:04:39 -0700

On 07/09, Lorenzo Bianconi wrote:
> On Jul 07, Stanislav Fomichev wrote:
> > On 07/03, Jesper Dangaard Brouer wrote:
> > > 
> > > 
> > > On 02/07/2025 18.05, Stanislav Fomichev wrote:
> > > > On 07/02, Jesper Dangaard Brouer wrote:
> > > > > This patch series introduces a mechanism for an XDP program to store RX
> > > > > metadata hints - specifically rx_hash, rx_vlan_tag, and rx_timestamp -
> > > > > into the xdp_frame. These stored hints are then used to populate the
> > > > > corresponding fields in the SKB that is created from the xdp_frame
> > > > > following an XDP_REDIRECT.
> > > > > 
> > > > > The chosen RX metadata hints intentionally map to the existing NIC
> > > > > hardware metadata that can be read via kfuncs [1]. While this design
> > > > > allows a BPF program to read and propagate existing hardware hints, our
> > > > > primary motivation is to enable setting custom values. This is important
> > > > > for use cases where the hardware-provided information is insufficient or
> > > > > needs to be calculated based on packet contents unavailable to the
> > > > > hardware.
> > > > > 
> > > > > The primary motivation for this feature is to enable scalable load
> > > > > balancing of encapsulated tunnel traffic at the XDP layer. When tunnelled
> > > > > packets (e.g., IPsec, GRE) are redirected via cpumap or to a veth device,
> > > > > the networking stack later calculates a software hash based on the outer
> > > > > headers. For a single tunnel, these outer headers are often identical,
> > > > > causing all packets to be assigned the same hash. This collapses all
> > > > > traffic onto a single RX queue, creating a performance bottleneck and
> > > > > defeating receive-side scaling (RSS).
> > > > > 
> > > > > Our immediate use case involves load balancing IPsec traffic. For such
> > > > > tunnelled traffic, any hardware-provided RX hash is calculated on the
> > > > > outer headers and is therefore incorrect for distributing inner flows.
> > > > > There is no reason to read the existing value, as it must be recalculated.
> > > > > In our XDP program, we perform a partial decryption to access the inner
> > > > > headers and calculate a new load-balancing hash, which provides better
> > > > > flow distribution. However, without this patch set, there is no way to
> > > > > persist this new hash for the network stack to use post-redirect.
> > > > > 
> > > > > This series solves the problem by introducing new BPF kfuncs that allow an
> > > > > XDP program to write e.g. the hash value into the xdp_frame. The
> > > > > __xdp_build_skb_from_frame() function is modified to use this stored value
> > > > > to set skb->hash on the newly created SKB. As a result, the veth driver's
> > > > > queue selection logic uses the BPF-supplied hash, achieving proper
> > > > > traffic distribution across multiple CPU cores. This also ensures that
> > > > > consumers, like the GRO engine, can operate effectively.
> > > > > 
> > > > > We considered XDP traits as an alternative to adding static members to
> > > > > struct xdp_frame. Given the immediate need for this functionality and the
> > > > > current development status of traits, we believe this approach is a
> > > > > pragmatic solution. We are open to migrating to a traits-based
> > > > > implementation if and when they become a generally accepted mechanism for
> > > > > such extensions.
> > > > > 
> > > > > [1] https://docs.kernel.org/networking/xdp-rx-metadata.html
> > > > > ---
> > > > > V1: https://lore.kernel.org/all/174897271826.1677018.9096866882347745168.stgit@firesoul/
> > > > 
> > > > No change log?
> > > 
> > > We have fixed selftest as requested by Alexie.
> > > And we have updated cover-letter and doc as you Stanislav requested.
> > > 
> > > > 
> > > > Btw, any feedback on the following from v1?
> > > > - https://lore.kernel.org/netdev/aFHUd98juIU4Rr9J@mini-arch/
> > > 
> > > Addressed as updated cover-letter and documentation. I hope this helps
> > > reviewers understand the use-case, as the discussion turn into "how do we
> > > transfer all HW metadata", which is NOT what we want (and a waste of
> > > precious cycles).
> > > 
> > > For our use-case, it doesn't make sense to "transfer all HW metadata".
> > > In fact we don't even want to read the hardware RH-hash, because we already
> > > know it is wrong (for tunnels), we just want to override the RX-hash used at
> > > SKB creation.  We do want the BPF programmers flexibility to call these
> > > kfuncs individually (when relevant).
> > > 
> > > > - https://lore.kernel.org/netdev/[email-masked]/
> > > 
> > > I feel pressured into critiquing Jakub's suggestion, hope this is not too
> > > harsh.  First of all it is not relevant to our this patchset use-case, as it
> > > focus on all HW metadata.
> > 
> > [..]
> > 
> > > Second, I disagree with the idea/mental model of storing in a
> > > "driver-specific format". The current implementation of driver-specific
> > > kfunc helpers that "get the metadata" is already doing a conversion to a
> > > common format, because the BPF-programmer naturally needs this to be the
> > > same across drivers.  Thus, it doesn't make sense to store it back in a
> > > "driver-specific format", as that just complicate things.  My mental model
> > > is thus, that after the driver-specific "get" operation to result is in a
> > > common format, that is simply defined by the struct type of the kfunc, which
> > > is both known by the kernel and BPF-prog.
> > 
> > Having get/set model seems a bit more generic, no? Potentially giving us the
> > ability to "correct" HW metadata for the non-redirected cases as well.
> > Plus we don't hard-code the (internal) layout. Solving only xdp_redirect
> > seems a bit too narrow, idk..
> 
> I can't see what the non-redirected use-case could be. Can you please provide
> more details?
> Moreover, can it be solved without storing the rx_hash (or the other
> hw-metadata) in a non-driver specific format?

Having setters feels more generic than narrowly solving only the redirect,
but I don't have a good use-case in mind.

> Storing the hw-metadata in some of hw-specific format in xdp_frame will not
> allow to consume them directly building the skb and we will require to decode
> them again. What is the upside/use-case of this approach? (not considering the
> orthogonality with the get method).

If we add the store kfuncs to regular drivers, the metadata  won't be stored
in the xdp_frame; it will go into the rx descriptors so regular path that
builds skbs will use it.