29.09.2026 13:44, Morten Brørup пишет:
From: Konstantin Ananyev [mailto:[email protected]]
Sent: Tuesday, 29 September 2026 14.00
28.09.2026 19:16, Randy L Tice пишет:
From: Randy L Tice <[email protected]>
Date: Thu, 03 Sep 2026 09:13:28 -0400
Add build-time support for optional cache-line-aligned dynamic-field
storage at the end of struct rte_mbuf.
The mbuf_dynfield3_size Meson option sets RTE_MBUF_DYNFIELD3_SIZE in
rte_build_config.h. A non-zero value enables the extra area and grows
every mbuf by the configured amount.
I am strongly opposed to that patch.
Inside mbuf we already do have priv_size that allows user to store
his/her specific
data straight after rte_mbuf in adjacent manner.
It worked well so far for many use-cases (including VPP) and I don't
see any
reason why this is not enough.
From other side - making size of core rte_mbuf configurable at run-
time,
will affect DPDK ABI stability in a negative way.
Fro my perspective it is much plausible in terms of ABI stability and
predictability
to have just one fixed layout for the mbuf.
Konstantin
The private data area (priv_size) is independent per mbuf pool, and selected at
run-time when creating each pool. As Randy explained in the RFC, this is
unavailable for mbuf pools created by other components.
I think it should be trivial to enforce minimal priv_size across all
mbuf pools what will be obeyed by different components
(as long as they do use rte_pktmbuf_pool_create() and friends):
1) introduce new EAL parameter 'mbuf-min-priv-size' or so (keep default
as zero)
2) make rte_pktmbuf_pool_create_by_ops() and
rte_pktmbuf_pool_create_extbuf() to check that input paramter
'priv_size' GE then value specified by EAL parameter, if so then return
an error.
Mbuf dynamic fields are shared across all mbuf pools, and serves the need with
an existing API. So I am strongly in favor of using the mbuf dynamic fields API
for this.
I agree with Konstantin that it would be optimal if the size of the added
dynfields area was run-time configurable (as an EAL startup parameter).
However, such a modification to the mbuf library would also require that the
performance cost in the dataplane is negligible. We don't want to compromise on
mbuf performance for applications not using this new feature.
Randy,
Could you please explore such an approach?
-Morten
