Re: [dpdk-dev] [PATCH] ethdev: extend flow metadata

[Adrien Mazarguil <adrien.mazarguil@6wind.com>]

On Wed, Jul 10, 2019 at 04:37:46PM +0000, Yongseok Koh wrote:
> 
> > On Jul 10, 2019, at 5:26 AM, Thomas Monjalon <thomas@monjalon.net> wrote:
> > 
> > 10/07/2019 14:01, Bruce Richardson:
> >> On Wed, Jul 10, 2019 at 12:07:43PM +0200, Olivier Matz wrote:
> >>> On Wed, Jul 10, 2019 at 10:55:34AM +0100, Bruce Richardson wrote:
> >>>> On Wed, Jul 10, 2019 at 11:31:56AM +0200, Olivier Matz wrote:
> >>>>> On Thu, Jul 04, 2019 at 04:21:22PM -0700, Yongseok Koh wrote:
> >>>>>> Currently, metadata can be set on egress path via mbuf tx_meatadata field
> >>>>>> with PKT_TX_METADATA flag and RTE_FLOW_ITEM_TYPE_RX_META matches metadata.
> >>>>>> 
> >>>>>> This patch extends the usability.
> >>>>>> 
> >>>>>> 1) RTE_FLOW_ACTION_TYPE_SET_META
> >>>>>> 
> >>>>>> When supporting multiple tables, Tx metadata can also be set by a rule and
> >>>>>> matched by another rule. This new action allows metadata to be set as a
> >>>>>> result of flow match.
> >>>>>> 
> >>>>>> 2) Metadata on ingress
> >>>>>> 
> >>>>>> There's also need to support metadata on packet Rx. Metadata can be set by
> >>>>>> SET_META action and matched by META item like Tx. The final value set by
> >>>>>> the action will be delivered to application via mbuf metadata field with
> >>>>>> PKT_RX_METADATA ol_flag.
> >>>>>> 
> >>>>>> For this purpose, mbuf->tx_metadata is moved as a separate new field and
> >>>>>> renamed to 'metadata' to support both Rx and Tx metadata.
> >>>>>> 
> >>>>>> For loopback/hairpin packet, metadata set on Rx/Tx may or may not be
> >>>>>> propagated to the other path depending on HW capability.
> >>>>>> 
> >>>>>> Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
> >>>>> 
> >>>>>> --- a/lib/librte_mbuf/rte_mbuf.h
> >>>>>> +++ b/lib/librte_mbuf/rte_mbuf.h
> >>>>>> @@ -648,17 +653,6 @@ struct rte_mbuf {
> >>>>>> 			/**< User defined tags. See rte_distributor_process() */
> >>>>>> 			uint32_t usr;
> >>>>>> 		} hash;                   /**< hash information */
> >>>>>> -		struct {
> >>>>>> -			/**
> >>>>>> -			 * Application specific metadata value
> >>>>>> -			 * for egress flow rule match.
> >>>>>> -			 * Valid if PKT_TX_METADATA is set.
> >>>>>> -			 * Located here to allow conjunct use
> >>>>>> -			 * with hash.sched.hi.
> >>>>>> -			 */
> >>>>>> -			uint32_t tx_metadata;
> >>>>>> -			uint32_t reserved;
> >>>>>> -		};
> >>>>>> 	};
> >>>>>> 
> >>>>>> 	/** Outer VLAN TCI (CPU order), valid if PKT_RX_QINQ is set. */
> >>>>>> @@ -727,6 +721,11 @@ struct rte_mbuf {
> >>>>>> 	 */
> >>>>>> 	struct rte_mbuf_ext_shared_info *shinfo;
> >>>>>> 
> >>>>>> +	/** Application specific metadata value for flow rule match.
> >>>>>> +	 * Valid if PKT_RX_METADATA or PKT_TX_METADATA is set.
> >>>>>> +	 */
> >>>>>> +	uint32_t metadata;
> >>>>>> +
> >>>>>> } __rte_cache_aligned;
> >>>>> 
> >>>>> This will break the ABI, so we cannot put it in 19.08, and we need a
> >>>>> deprecation notice.
> >>>>> 
> >>>> Does it actually break the ABI? Adding a new field to the mbuf should only
> >>>> break the ABI if it either causes new fields to move or changes the
> >>>> structure size. Since this is at the end, it's not going to move any older
> >>>> fields, and since everything is cache-aligned I don't think the structure
> >>>> size changes either.
> >>> 
> >>> I think it does break the ABI: in previous version, when the PKT_TX_METADATA
> >>> flag is set, the associated value is put in m->tx_metadata (offset 44 on
> >>> x86-64), and in the next version, it will be in m->metadata (offset 112). So,
> >>> these 2 versions are not binary compatible.
> >>> 
> >>> Anyway, at least it breaks the API.
> >> 
> >> Ok, I misunderstood. I thought it was the structure change itself you were
> >> saying broke the ABI. Yes, putting the data in a different place is indeed
> >> an ABI break.
> > 
> > We could add the new field and keep the old one unused,
> > so it does not break the ABI.
> 
> Still breaks ABI if PKT_TX_METADATA is set. :-) In order not to break it, I can
> keep the current union'd field (tx_metadata) as is with PKT_TX_METADATA, add
> the new one at the end and make it used with the new PKT_RX_METADATA.
> 
> > However I suppose everybody will prefer a version using dynamic fields.
> > Is someone against using dynamic field for such usage?
> 
> However, given that the amazing dynamic fields is coming soon (thanks for your
> effort, Olivier and Thomas!), I'd be honored to be the first user of it.
> 
> Olivier, I'll take a look at your RFC.

Just got a crazy idea while reading this thread... How about repurposing
that "reserved" field as "rx_metadata" in the meantime?

I know reserved fields are cursed and no one's ever supposed to touch them
but this risk is mitigated by having the end user explicitly request its
use, so the patch author (and his relatives) should be safe from the
resulting bad juju.

Joke aside, while I like the idea of Tx/Rx META, I think the similarities
with MARK (and TAG eventually) is a problem. I wasn't available and couldn't
comment when META was originally added to the Tx path, but there's a lot of
overlap between these items/actions, without anything explaining to the end
user how and why they should pick one over the other, if they can be
combined at all and what happens in that case.

All this must be documented, then we should think about unifying their
respective features and deprecate the less capable items/actions. In my
opinion, users need exactly one method to mark/match some mark while
processing Rx/Tx traffic and *optionally* have that mark read from/written
to the mbuf, which may or may not be possible depending on HW features.

-- 
Adrien Mazarguil
6WIND