Felix Fietkau <nbd@xxxxxxxx> writes: > On 14.03.22 21:39, Jesper D. Brouer wrote: >> (Cc. BPF list and other XDP maintainers) >> >> On 14/03/2022 11.22, Felix Fietkau wrote: >>> Most ethernet drivers allocate a packet headroom of NET_SKB_PAD. Since it is >>> rounded up to L1 cache size, it ends up being at least 64 bytes on the most >>> common platforms. >>> On most ethernet drivers, having a guaranteed headroom of 256 bytes for XDP >>> adds an extra forced pskb_expand_head call when enabling SKB XDP, which can >>> be quite expensive. >>> Many XDP programs need only very little headroom, so it can be beneficial >>> to have a way to opt-out of the 256 bytes headroom requirement. >> >> IMHO 64 bytes is too small. >> We are using this area for struct xdp_frame and also for metadata >> (XDP-hints). This will limit us from growing this structures for >> the sake of generic-XDP. >> >> I'm fine with reducting this to 192 bytes, as most Intel drivers >> have this headroom, and have defacto established that this is >> a valid XDP headroom, even for native-XDP. >> >> We could go a small as two cachelines 128 bytes, as if xdp_frame >> and metadata grows above a cache-line (64 bytes) each, then we have >> done something wrong (performance wise). > Here's some background on why I chose 64 bytes: I'm currently > implementing a userspace + xdp program to act as generic fastpath to > speed network bridging. Any reason this can't run in the TC ingress hook instead? Generic XDP is a bit of an odd duck, and I'm not a huge fan of special-casing it this way... -Toke
