From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id A087E43E36; Wed, 10 Apr 2024 14:20:50 +0200 (CEST) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 254B0402CF; Wed, 10 Apr 2024 14:20:50 +0200 (CEST) Received: from dkmailrelay1.smartsharesystems.com (smartserver.smartsharesystems.com [77.243.40.215]) by mails.dpdk.org (Postfix) with ESMTP id 65BA1402C7; Wed, 10 Apr 2024 14:20:48 +0200 (CEST) Received: from smartserver.smartsharesystems.com (smartserver.smartsharesys.local [192.168.4.10]) by dkmailrelay1.smartsharesystems.com (Postfix) with ESMTP id 225222274F; Wed, 10 Apr 2024 14:20:48 +0200 (CEST) Content-class: urn:content-classes:message MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Subject: RE: [PATCH v2 3/8] mbuf: fix Tx checksum offload examples Date: Wed, 10 Apr 2024 14:20:42 +0200 X-MimeOLE: Produced By Microsoft Exchange V6.5 Message-ID: <98CBD80474FA8B44BF855DF32C47DC35E9F38F@smartserver.smartshare.dk> In-Reply-To: <409157f5da3e4c628ca678dd9e2c7957@huawei.com> X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: [PATCH v2 3/8] mbuf: fix Tx checksum offload examples Thread-Index: AQHah2gcwh+zxBWPrEOmSW2nVSxrXbFZuegAgAX4BtCAAE6MMIABTCywgAAbf0A= References: <20240405125039.897933-1-david.marchand@redhat.com> <20240405144604.906695-1-david.marchand@redhat.com> <20240405144604.906695-4-david.marchand@redhat.com> <98CBD80474FA8B44BF855DF32C47DC35E9F36C@smartserver.smartshare.dk> <10b564b42f8d4db387f6302701f24ce3@huawei.com> <98CBD80474FA8B44BF855DF32C47DC35E9F381@smartserver.smartshare.dk> <409157f5da3e4c628ca678dd9e2c7957@huawei.com> From: =?iso-8859-1?Q?Morten_Br=F8rup?= To: "Konstantin Ananyev" , "David Marchand" , Cc: , , , "Olivier Matz" , "Jijiang Liu" , "Andrew Rybchenko" , "Ferruh Yigit" , "Kaiwen Deng" , , , "Aman Singh" , "Yuying Zhang" , "Thomas Monjalon" , "Jerin Jacob" X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com] > Sent: Wednesday, 10 April 2024 12.35 >=20 > > > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com] > > > Sent: Tuesday, 9 April 2024 15.39 > > > > > > > > From: David Marchand [mailto:david.marchand@redhat.com] > > > > > Sent: Friday, 5 April 2024 16.46 > > > > > > > > > > Mandate use of rte_eth_tx_prepare() in the mbuf Tx checksum = offload > > > > > examples. > > > > > > > > I strongly disagree with this change! > > > > > > > > It will cause a huge performance degradation for shaping = applications: > > > > > > > > A packet will be processed and finalized at an output or = forwarding > > > pipeline stage, where some other fields might also be written, so > > > > zeroing e.g. the out_ip checksum at this stage has low cost (no = new > > > cache misses). > > > > > > > > Then, the packet might be queued for QoS or similar. > > > > > > > > If rte_eth_tx_prepare() must be called at the egress pipeline = stage, > > > it has to write to the packet and cause a cache miss per packet, > > > > instead of simply passing on the packet to the NIC hardware. > > > > > > > > It must be possible to finalize the packet at the = output/forwarding > > > pipeline stage! > > > > > > If you can finalize your packet on output/forwarding, then why = you > > > can't invoke tx_prepare() on the same stage? > > > There seems to be some misunderstanding about what tx_prepare() = does - > > > in fact it doesn't communicate with HW queue (doesn't update TXD = ring, > > > etc.), what it does - just make changes in mbuf itself. > > > Yes, it reads some fields in SW TX queue struct (max number of = TXDs per > > > packet, etc.), but AFAIK it is safe > > > to call tx_prepare() and tx_burst() from different threads. > > > At least on implementations I am aware about. > > > Just checked the docs - it seems not stated explicitly anywhere, = might > > > be that's why it causing such misunderstanding. > > > > > > > > > > > Also, how is rte_eth_tx_prepare() supposed to work for cloned = packets > > > egressing on different NIC hardware? > > > > > > If you create a clone of full packet (including L2/L3) headers = then > > > obviously such construction might not > > > work properly with tx_prepare() over two different NICs. > > > Though In majority of cases you do clone segments with data, while = at > > > least L2 headers are put into different segments. > > > One simple approach would be to keep L3 header in that separate = segment. > > > But yes, there is a problem when you'll need to send exactly the = same > > > packet over different NICs. > > > As I remember, for bonding PMD things don't work quite well here - = you > > > might have a bond over 2 NICs with > > > different tx_prepare() and which one to call might be not clear = till > > > actual PMD tx_burst() is invoked. > > > > > > > > > > > In theory, it might get even worse if we make this opaque = instead of > > > transparent and standardized: > > > > One PMD might reset out_ip checksum to 0x0000, and another PMD = might > > > reset it to 0xFFFF. > > > > > > > > > > > I can only see one solution: > > > > We need to standardize on common minimum requirements for how to > > > prepare packets for each TX offload. > > > > > > If we can make each and every vendor to agree here - that = definitely > > > will help to simplify things quite a bit. > > > > An API is more than a function name and parameters. > > It also has preconditions and postconditions. > > > > All major NIC vendors are contributing to DPDK. > > It should be possible to reach consensus for reasonable minimum = requirements > for offloads. > > Hardware- and driver-specific exceptions can be documented with the = offload > flag, or with rte_eth_rx/tx_burst(), like the note to > > rte_eth_rx_burst(): > > "Some drivers using vector instructions require that nb_pkts is = divisible by > 4 or 8, depending on the driver implementation." >=20 > If we introduce a rule that everyone supposed to follow and then = straightway > allow people to have a 'documented exceptions', > for me it means like 'no rule' in practice. > A 'documented exceptions' approach might work if you have 5 different = PMDs to > support, but not when you have 50+. > No-one would write an app with possible 10 different exception cases = in his > head. > Again, with such approach we can forget about backward compatibility. > I think we already had this discussion before, my opinion remains the = same > here - > 'documented exceptions' approach is a way to trouble. The "minimum requirements" should be the lowest common denominator of = all NICs. Exceptions should be extremely few, for outlier NICs that still want to = provide an offload and its driver is unable to live up to the minimum = requirements. Any exception should require techboard approval. If a NIC/driver does = not support the "minimum requirements" for an offload feature, it is not = allowed to claim support for that offload feature, or needs to seek = approval for an exception. As another option for NICs not supporting the minimum requirements of an = offload feature, we could introduce offload flags with finer = granularity. E.g. one offload flag for "gold standard" TX checksum = update (where the packet's checksum field can have any value), and = another offload flag for "silver standard" TX checksum update (where the = packet's checksum field must have a precomputed value). For reference, consider RSS, where the feature support flags have very = high granularity. >=20 > > You mention the bonding driver, which is a good example. > > The rte_eth_tx_burst() documentation has a note about the API = postcondition > exception for the bonding driver: > > "This function must not modify mbufs (including packets data) unless = the > refcnt is 1. An exception is the bonding PMD, [...], mbufs > > may be modified." >=20 > For me, what we've done for bonding tx_prepare/tx_burst() is a really = bad > example. > Initial agreement and design choice was that tx_burst() should not = modify > contents of the packets > (that actually was one of the reasons why tx_prepare() was = introduced). > The only reason I agreed on that exception - because I couldn't = come-up with > something less uglier. >=20 > Actually, these problems with bonding PMD made me to start thinking = that > current > tx_prepare/tx_burst approach might need to be reconsidered somehow. In cases where a preceding call to tx_prepare() is required, how is it = worse modifying the packet in tx_burst() than modifying the packet in = tx_prepare()? Both cases violate the postcondition that packets are not modified at = egress. >=20 > > > Then we can probably have one common tx_prepare() for all vendors = ;) > > > > Yes, that would be the goal. > > More realistically, the ethdev layer could perform the common = checks, and > only the non-conforming drivers would have to implement > > their specific tweaks. >=20 > Hmm, but that's what we have right now: > - fields in mbuf and packet data that user has to fill correctly and = dev > specific tx_prepare(). > How what you suggest will differ then? You're 100 % right here. We could move more checks into the ethdev = layer, specifically checks related to the "minimum requirements". > And how it will help let say with bonding PMD situation, or with = TX-ing of the > same packet over 2 different NICs? The bonding driver is broken. It can only be fixed by not violating the egress postcondition in either = tx_burst() or tx_prepare(). "Minimum requirements" might help doing that. >=20 > > If we don't standardize the meaning of the offload flags, the = application > developers cannot trust them! > > I'm afraid this is the current situation - application developers = either > test with specific NIC hardware, or don't use the offload features. >=20 > Well, I have used TX offloads through several projects, it worked = quite well. That is good to hear. And I don't oppose to that. In this discussion, I am worried about the roadmap direction for DPDK. I oppose to the concept of requiring calling tx_prepare() before calling = tx_burst() when using offload. I think it is conceptually wrong, and = breaks the egress postcondition. I propose "minimum requirements" as a better solution. > Though have to admit, never have to use TX offloads together with our = bonding > PMD. >=20