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/20250616145523.63bd2577@xxxxxxxxxx/ > > 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..
