Re: [PATCH 9/9] net/dpaa2: drop the fake software VLAN strip offload

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 11 Jun 2026 17:49:24 +0200
Maxime Leroy <maxime@leroys.fr> wrote:

> 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>
> ---

In general I agree, but you overstate the impact. Any real application
is going to look at the mbuf anyway. Relying on testpmd numbers is BS.

The NBL driver does the same thing.
So does PCAP but it has no choice, and is slow anyway.
Virtio/vhost does as well.