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 1F87543E6F; Tue, 16 Apr 2024 11:16:57 +0200 (CEST) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id BF8A0402E4; Tue, 16 Apr 2024 11:16:53 +0200 (CEST) Received: from frasgout.his.huawei.com (frasgout.his.huawei.com [185.176.79.56]) by mails.dpdk.org (Postfix) with ESMTP id ECA73402E3; Tue, 16 Apr 2024 11:16:52 +0200 (CEST) Received: from mail.maildlp.com (unknown [172.18.186.216]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4VJdfR0XhRz6D99x; Tue, 16 Apr 2024 17:14:55 +0800 (CST) Received: from frapeml500008.china.huawei.com (unknown [7.182.85.71]) by mail.maildlp.com (Postfix) with ESMTPS id 1E46C1400D9; Tue, 16 Apr 2024 17:16:52 +0800 (CST) Received: from frapeml500007.china.huawei.com (7.182.85.172) by frapeml500008.china.huawei.com (7.182.85.71) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.1.2507.35; Tue, 16 Apr 2024 11:16:51 +0200 Received: from frapeml500007.china.huawei.com ([7.182.85.172]) by frapeml500007.china.huawei.com ([7.182.85.172]) with mapi id 15.01.2507.035; Tue, 16 Apr 2024 11:16:51 +0200 From: Konstantin Ananyev To: =?iso-8859-1?Q?Morten_Br=F8rup?= , "David Marchand" , "dev@dpdk.org" CC: "thomas@monjalon.net" , "ferruh.yigit@amd.com" , "stable@dpdk.org" , Olivier Matz , Jijiang Liu , "Andrew Rybchenko" , Ferruh Yigit , Kaiwen Deng , "qiming.yang@intel.com" , "yidingx.zhou@intel.com" , Aman Singh , "Yuying Zhang" , Thomas Monjalon , "Jerin Jacob" Subject: RE: [PATCH v2 3/8] mbuf: fix Tx checksum offload examples Thread-Topic: [PATCH v2 3/8] mbuf: fix Tx checksum offload examples Thread-Index: AQHah2gcwh+zxBWPrEOmSW2nVSxrXbFZuegAgAX4BtCAAE6MMIABTCywgAAbf0CAAzI3UIAAC0aQgAAit/CAAAXFUIAF1aYQ Date: Tue, 16 Apr 2024 09:16:51 +0000 Message-ID: <3f8214e0bcb448338cf2679f753a983d@huawei.com> 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> <98CBD80474FA8B44BF855DF32C47DC35E9F38F@smartserver.smartshare.dk> <52850a78c83445548a0b78bfd04e6f91@huawei.com> <98CBD80474FA8B44BF855DF32C47DC35E9F39F@smartserver.smartshare.dk> <98CBD80474FA8B44BF855DF32C47DC35E9F3A0@smartserver.smartshare.dk> In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9F3A0@smartserver.smartshare.dk> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.206.138.42] Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 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 > > > > > > > > > > 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 updat= e > > 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 exactl= y > > 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 anothe= r > > 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." > > > > > > > > > > > > If we introduce a rule that everyone supposed to follow and the= n > > > > 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 denominato= r > > 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 requirement= s > > 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 checksu= m > > > > update (where the packet's checksum field must have a > > > > > precomputed value). > > > > > > > > Actually yes, I was thinking in the same direction - we need some > > extra > > > > API to allow user to distinguish. > > > > Probably we can do something like that: a new API for the ethdev > > call > > > > that would take as a parameter > > > > TX offloads bitmap and in return specify would it need to modify > > > > contents of packet to support these > > > > offloads or not. > > > > Something like: > > > > int rte_ethdev_tx_offload_pkt_mod_required(unt64_t tx_offloads) > > > > > > > > For the majority of the drivers that satisfy these "minimum > > > > requirements" corresponding devops > > > > entry will be empty and we'll always return 0, otherwise PMD has to > > > > provide a proper devop. > > > > Then again, it would be up to the user, to determine can he pass > > same > > > > packet to 2 different NICs or not. > > > > > > > > I suppose it is similar to what you were talking about? > > > > > > I was thinking something more simple: > > > > > > The NIC exposes its RX and TX offload capabilities to the application > > through the rx/tx_offload_capa and other fields in the > > > rte_eth_dev_info structure returned by rte_eth_dev_info_get(). > > > > > > E.g. tx_offload_capa might have the RTE_ETH_TX_OFFLOAD_IPV4_CKSUM fla= g > > set. > > > These capability flags (or enums) are mostly undocumented in the code= , > > but I guess that the RTE_ETH_TX_OFFLOAD_IPV4_CKSUM > > > capability means that the NIC is able to update the IPv4 header > > checksum at egress (on the wire, i.e. without modifying the mbuf or > > > packet data), and that the application must set RTE_MBUF_F_TX_IP_CKSU= M > > in the mbufs to utilize this offload. > > > I would define and document what each capability flag/enum exactly > > means, the minimum requirements (as defined by the DPDK > > > community) for the driver to claim support for it, and the > > requirements for an application to use it. > > > For the sake of discussion, let's say that > > RTE_ETH_TX_OFFLOAD_IPV4_CKSUM means "gold standard" TX checksum update > > capability > > > (i.e. no requirements to the checksum field in the packet contents). > > > If some NIC requires the checksum field in the packet contents to hav= e > > a precomputed value, the NIC would not be allowed to claim > > > the RTE_ETH_TX_OFFLOAD_IPV4_CKSUM capability. > > > Such a NIC would need to define and document a new capability, e.g. > > RTE_ETH_TX_OFFLOAD_IPV4_CKSUM_ASSISTED, for the "silver > > > standard" TX checksum update capability. > > > In other words: I would encode variations of offload capabilities > > directly in the capabilities flags. > > > Then we don't need additional APIs to help interpret those > > capabilities. > > > > I understood your intention with different flags, yes it should work to= o > > I think. > > The reason I am not very fond of it - it will require to double > > TX_OFFLOAD flags. >=20 > An additional feature flag is only required if a NIC is not conforming to= the "minimum requirements" of an offload feature, and the > techboard permits introducing a variant of an existing feature. > There should be very few additional feature flags for variants - exceptio= ns only - or the "minimum requirements" are not broad > enough to support the majority of NICs. Ok, so you suggest to group all existing reqs plus what all current tx_prep= are() do into "minimum requirements"? So with current drivers in place we wouldn't need these new flags, but we'l= l reserve such opportunity. That might work, if there are no contradictory requirements in current PMDs= , and PMDs maintainers with less reqs will agree with these 'extra' stuff. =20 =20 > > > > > This way, the application can probe the NIC capabilities to determine > > what can be offloaded, and how to do it. > > > > > > The application can be designed to: > > > 1. use a common packet processing pipeline, utilizing only the lowest > > common capabilities denominator of all detected NICs, or > > > 2. use a packet processing pipeline, handling packets differently > > according to the capabilities of the involved NICs. > > > > > > NB: There may be other variations than requiring packet contents to b= e > > modified, and they might be granular. > > > E.g. a NIC might require assistance for TCP/UDP checksum offload, but > > not for IP checksum offload, so a function telling if packet > > > contents requires modification would not suffice. > > > > Why not? > > If user plans to use multiple tx offloads provide a bitmask of all of > > them as an argument. > > Let say for both L3 and L4 cksum offloads it will be something like: > > (RTE_ETH_TX_OFFLOAD_IPV4_CKSUM | RTE_ETH_TX_OFFLOAD_UDP_CKSUM | > > RTE_ETH_TX_OFFLOAD_TCP_CKSUM) >=20 > You are partially right; the offload flags can be tested one by one to de= termine which header fields in the packet contents need to be > updated. Why one by one? I think user will be able to test all TX offloads he needs at once.=20 What user needs to know - can he expect that for tx offloads he selected PM= D tx_burst will not modify the packet and metadata. If the answer is 'yes' then he can safely TX the same mbuf over different P= MDs simultaneously. If the answer is 'no' then he either has to avoid TX offloads that cause mo= difications or be prepared to overcome that situation: copy packet, or might be just copy only inner/outer L2/L3/L4 headers into a= separate segment, etc. =20 > I'm assuming the suggested function returns a Boolean; so it doesn't tell= the application how it should modify the packet contents, e.g. > if the header's checksum field must be zeroed or contain the precomputed = checksum of the pseudo header. > Alternatively, if it returns a bitfield with flags for different types of= modification, the information could be encoded that way. That seems an unnecessary overcomplication to me and probably not necessary= , see above what was my thought. =20 > But then > they might as well be returned as offload capability variation flags in t= he rte_eth_dev_info structure's tx_offload_capa field, as I'm > advocating for. >=20 > > > > > E.g. RTE_ETH_TX_OFFLOAD_MULTI_SEGS is defined, but the > > rte_eth_dev_info structure doesn't expose information about the max > > > number of segments it can handle. > > > > > > PS: For backwards compatibility, we might define > > RTE_ETH_TX_OFFLOAD_IPV4_CKSUM as the "silver standard" offload to > > support the > > > current "minimum requirements", and add > > RTE_ETH_TX_OFFLOAD_IPV4_CKSUM_ANY for the "gold standard" offload. > > > > > > > > > > > > > > > For reference, consider RSS, where the feature support flags have > > very > > > > high granularity. > > > > > > > > > > > > > > > > > > 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." > > > > > > > > > > > > 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. > > > > > > > > > > > > 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 modifie= d > > at > > > > egress. > > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > Hmm, but that's what we have right now: > > > > > > - fields in mbuf and packet data that user has to fill correctl= y > > and > > > > dev > > > > > > specific tx_prepare(). > > > > > > How what you suggest will differ then? > > > > > > > > > > You're 100 % right here. We could move more checks into the ethde= v > > > > layer, specifically checks related to the "minimum > > > > > requirements". > > > > > > > > > > > And how it will help let say with bonding PMD situation, or wit= h > > 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. > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > 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. > > > > > >