Re: [PATCH v2 6/6] net/dpaa2: drop the fake software VLAN strip offload

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 16 Jun 2026 12:27:26 +0200
Maxime Leroy <maxime@leroys.fr> wrote:

> RTE_ETH_RX_OFFLOAD_VLAN_STRIP is advertised, but no hardware VLAN strip
> backs it: when enabled, the Rx burst calls rte_vlan_strip() on every
> frame, a software op masquerading as a hardware offload.
> 
> It saves a forwarding application nothing: the datapath reads the L2
> header anyway to classify or strip. The offload does not remove that
> read, it relocates it into the driver Rx burst, where it is far more
> expensive.
> 
> The cost is a matter of timing. rte_vlan_strip() reaches the L2 header
> through rte_pktmbuf_mtod(), which dereferences mbuf->buf_addr. On a
> freshly recycled buffer that mbuf cacheline is cold. eth_fd_to_mbuf()
> has just written other fields of it (data_off, ol_flags), but buf_addr
> is a persistent field it does not rewrite. A write does not stall: it
> posts to the store buffer while the line fills in the background, and
> the rewritten fields are forwarded straight from there. buf_addr has
> nothing to forward, so it must be read from the line, whose fill is
> still in flight, and the read stalls. The ethertype read that follows,
> on the cold payload line, stalls again. Read later by the application,
> when the fill has completed, the same read hits. The offload just
> performs it at the worst possible moment.
> 
> Measured on a single-core port-to-port forwarding test over two 10G
> ports (one core at 2 GHz, 64-byte untagged frames):
> 
>   - throughput 4.22 -> 5.00 Mpps (+18 percent)
>   - IPC 0.93 -> 1.25: the cost was memory stall, not compute
>   - L3/DRAM-bound L2 refills 319M -> 200M over 10s (-37 percent)
> 
> perf confirms it: with the offload, the buf_addr load (the cold mbuf
> field) and the payload load account for about 84 percent of the Rx
> burst's L2 refills; removing it, those vanish and only the inherent DQRR
> dequeue misses remain.
> 
> Stop advertising VLAN_STRIP and remove the rte_vlan_strip() calls from
> every Rx path. This is a behavioural change: the tag is left in the
> frame, so an application must strip it itself, on the L2 header it
> already reads.
> 
> Signed-off-by: Maxime Leroy <maxime@leroys.fr>

Acked-by: Stephen Hemminger <stephen@networkplumber.org>