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 B89EA43B9D; Fri, 23 Feb 2024 09:37:05 +0100 (CET) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 898974068A; Fri, 23 Feb 2024 09:37:05 +0100 (CET) Received: from shelob.oktetlabs.ru (shelob.oktetlabs.ru [91.220.146.113]) by mails.dpdk.org (Postfix) with ESMTP id E3579402ED; Fri, 23 Feb 2024 09:37:03 +0100 (CET) Received: from [192.168.1.38] (unknown [188.170.76.194]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by shelob.oktetlabs.ru (Postfix) with ESMTPSA id 153BC2F; Fri, 23 Feb 2024 11:37:02 +0300 (MSK) DKIM-Filter: OpenDKIM Filter v2.11.0 shelob.oktetlabs.ru 153BC2F DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=oktetlabs.ru; s=default; t=1708677423; bh=gQRqCc+oDXfGUNLZat4aqTBBBuKyk9uxvGbGGYVkN8s=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=tyy+3vwS5XUQ0y5DXkfc2cRMVvfIabdwYeGtNAQDt1t/oR43O7nVABcC6xCDqAlNT Csg6x3HrFB1MN26doSsR1oF9G1rHEpmU/2BscISTXsKKQdr7RC/sr2DzcBI4KV0OU+ LNKY7LnMdaIrZQiksk0BAVRxgtvkns96ZZ6mhuNI= Message-ID: <545e4c08-f4cf-4246-9ed3-22882c754640@oktetlabs.ru> Date: Fri, 23 Feb 2024 11:36:54 +0300 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] app/testpmd: use Tx preparation in txonly engine To: Konstantin Ananyev , =?UTF-8?Q?Morten_Br=C3=B8rup?= , Ferruh Yigit , Kaiwen Deng , "dev@dpdk.org" Cc: "stable@dpdk.org" , "qiming.yang@intel.com" , "yidingx.zhou@intel.com" , Aman Singh , Yuying Zhang , David Marchand , Thomas Monjalon , Jerin Jacob References: <20240103012912.4334-1-kaiwenx.deng@intel.com> <20240111052555.35930-1-kaiwenx.deng@intel.com> <38baef9c-103a-4be4-8546-e9ea35abc46f@amd.com> <98CBD80474FA8B44BF855DF32C47DC35E9F1FE@smartserver.smartshare.dk> <918593c56c5745a285facc47b6cdc76b@huawei.com> <98CBD80474FA8B44BF855DF32C47DC35E9F20C@smartserver.smartshare.dk> <670ee735254244878e4b1247a8850be5@huawei.com> Content-Language: en-US From: Andrew Rybchenko In-Reply-To: <670ee735254244878e4b1247a8850be5@huawei.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 On 2/22/24 21:28, Konstantin Ananyev wrote: > >> +CC: Ethernet API maintainers >> +CC: Jerin (commented on another branch of this thread) >> >>> From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com] >>> Sent: Sunday, 11 February 2024 16.04 >>> >>>>>>> TSO breaks when MSS spans more than 8 data fragments. Those >>>>>>> packets will be dropped by Tx preparation API, but it will >>> cause >>>>>>> MDD event if txonly forwarding engine does not call the Tx >>>>> preparation >>>>>>> API before transmitting packets. >>>>>>> >>>>>> >>>>>> txonly is used commonly, adding Tx prepare for a specific case >>> may >>>>>> impact performance for users. >>>>>> >>>>>> What happens when driver throws MDD (Malicious Driver Detection) >>>>> event, >>>>>> can't it be ignored? As you are already OK to drop the packet, >>> can >>>>>> device be configured to drop these packages? >>>>>> >>>>>> >>>>>> Or as Jerin suggested adding a new forwarding engine is a >>> solution, >>>>> but >>>>>> that will create code duplication, I prefer to not have it if >>> this >>>>> can >>>>>> be handled in device level. >>>>> >>>>> Actually I am agree with the author of the patch - when TX offloads >>>>> and/or multisegs are enabled, >>>>> user supposed to invoke eth_tx_prepare(). >>>>> Not doing that seems like a bug to me. >>>> >>>> I strongly disagree with that statement, Konstantin! >>>> It is not documented anywhere that using TX offloads and/or multisegs >>> requires calling rte_eth_tx_prepare() before >>>> rte_eth_tx_burst(). And none of the examples do it. >>> >>> In fact, we do use it for test-pmd/csumonly.c. >>> About other sample apps: >>> AFAIK, not many of other DPDK apps do use L4 offloads. >>> Right now special treatment (pseudo-header cksum calculation) is needed >>> only for L4 offloads (CKSUM, SEG). >>> So, majority of our apps who rely on other TX offloads (multi-seg, ipv4 >>> cksum, vlan insertion) happily run without >>> calling tx_prepare(), even though it is not the safest way. >>> >>>> >>>> In my opinion: >>>> If some driver has limitations for a feature, e.g. max 8 fragments, >>> it should be documented for that driver, so the application >>>> developer can make the appropriate decisions when designing the >>> application. >>>> Furthermore, we have APIs for the drivers to expose to the >>> applications what the driver supports, so the application can configure >>>> itself optimally at startup. Perhaps those APIs need to be expanded. >>>> And if a feature limitation is common across the majority of drivers, >>> that limitation should be mentioned in the documentation of the >>>> feature itself. >>> >>> Many of such limitations *are* documented and in fact we do have an API >>> to check max segments that each driver support, >>> see struct rte_eth_desc_lim. >> >> Yes, this is the kind of API we should provide, so the application can configure itself appropriately. >> >>> The problem is: >>> - none of our sample app does proper check on these values, so users >>> don't have a good example how to do it. >> >> Agreed. >> Adding an example showing how to do it properly would be the best solution. >> Calling tx_prepare() in the examples is certainly not the solution. >> >>> - with current DPDK API not all of HW/PMD requirements could be >>> extracted programmatically: >>> let say majority of Intel PMDs for TCP offloads expect pseudo-header >>> cksum to be pre-calculated by the SW. >> >> I hope this requirement is documented somewhere. >> >>> another example, some HW expects pkt_len to be bigger then some >>> threshold value, otherwise HW hang may appear. >> >> I hope this requirement is also documented somewhere. > > No idea, I found it only in the code. IMHO Tx burst must check such limitations. If you made your HW simpler (or just lost it on initial testing), pay in your drivers (or your HW+driver will be unusable because of such problems). >> Generally, if the requirements cannot be extracted programmatically, they must be prominently documented, like this note to >> rte_eth_rx_burst(): > > Obviously, more detailed documentation is always good, but... > Right now we have 50+ different PMDs from different vendors. > Even if each and every of them will carefully document all possible limitations and necessary preparation steps, > how DPDK app developer supposed to deal with all that? > Do you expect everyone, to read carefully through all of them, and handle all of them properly oh his own > in each and every DPDK app he is going to write? > That seems unrealistic. > Again what to do with backward compatibility: when new driver (with new limitations) will arise > *after* your app is already written and tested? +1 >> >> * @note >> * Some drivers using vector instructions require that *nb_pkts* is >> * divisible by 4 or 8, depending on the driver implementation. I'm wondering what application should do if it needs to send just one packet and do it now. IMHO, such limitations are not acceptable. >> >>> - As new HW and PMD keep appearing it is hard to predict what extra >>> limitations/requirements will arise, >>> that's why tx_prepare() was introduced as s driver op. >>> >>>> >>>> We don't want to check in the fast path what can be checked at >>> startup or build time! >>> >>> If your app supposed to work with just a few, known in advance, NIC >>> models, then sure, you can do that. >>> For apps that supposed to work 'in general' with any possible PMDs >>> that DPDK supports - that might be a problem. >>> That's why tx_prepare() was introduced and it is strongly recommended >>> to use it by the apps that do use TX offloads. >>> Probably tx_prepare() is not the best possible approach, but right now >>> there are not many alternatives within DPDK. >> >> What exactly is an application supposed to do if tx_prepare() doesn't accept the full burst? It doesn't return information about what is >> wrong. > > It provides some information, but it is *very* limited: just index of 'bad' packet and error code. > In theory app can try to handle it in a 'smart' way: let say if ENOTSUP is returned, then try to disable all HW offloads > and do all in SW. But again, it is much better to do so *before* submitting packets for TX, so in practice everyone > just drop such 'bad' packets. > > Dropping the packets might not be an option, e.g. for applications used in life support or tele-medicine. Critical applications should be able to do all Tx offloads in SW and retry. Of course, various statistics and logs should help to improve the application. > If the packet is 'bad', then it is much better to drop it, then TX corrupted packet or even hang NIC HW completely. IMHO Tx burst must drop packet which could hang NIC HW completely. I realize that it is an extra checks and performance drop, but vendor should pay in performance if HW is not good enough. > Though off-course it is much better to have an app that would check for limitations that can be checked by API provided > and enable only supported offloads. Yes, that's why API to get limitation is much better than documentation. >> If limitations are documented, an application can use the lowest common denominator of the NICs it supports. And if the application is >> supposed to work in general, that becomes the lowest common denominator of all NICs. > > I agree: for limitations that can be extracted with generic API, like: > number of segments per packet, supported TX offloads, mbuf fileds that must be provided for each TX offload, etc. - > it is responsibility of well-written application to obey all of them. > Yes, many tx_prepare() implementations do such checks anyway, but from my perspective it is sort of last-line of defense. > For well written application that should just never happen. > But there is one more important responsibility of tx_prepare() - it performs PMD specific packet modifications for requested offloads. > As I already mentioned for Intel NICs - it does pseudo-header cksum calcucation, for packets with size less then minimal, it can > probably do padding (even if doesn't do it right now), for some other PMDs - might be something else, I didn't check. > Obviously it saves app developer from a burden to do all these things on his own. > >> It looks like tx_prepare() has become a horrible workaround for undocumented limitations. I strongly disagree. Documentation is never a solution for a generic application which is intended to work on any HW and IMHO it is the goal to have more and more applications which work on any HW. >> Limitations due to hardware and/or software tradeoffs are unavoidable, so we have to live with them; but we should not accept >> PMDs with undocumented limitations. > > As I already said, more documentation never hurts, but for that case, I think it is not enough. > I expect PMD to provide a tx_prepare() implementation that would deal with such specific things. +1 > Anyway, back to the original patch - I looked at it once again, and realized that the problem > is just in the unsupported number of segments. > As we discussed above - such limitations should be handled by well written app, > but none of DPDK apps does it right now. > So probably it is good opportunity to do things in a proper way and introduce such checks > in testpmd ;) +1 since we already have fields in device information to report such limitations, but it does not say that Tx prepare should be dropped. Drivers which don't need Tx prepare keep it NULL and it returns immediately from ethdev. Since it is done per-burst, it should not affect performance a lot. > Kaiwen, WDYT? > >>> >>>> >>>>> If it still works for some cases, that's a lucky coincidence, but not >>>>> the expected behavior. >>>>> About performance - first we can check is that really a drop. >>>>> Also as I remember most drivers set it to non-NULL value, only when >>>>> some TX offloads were >>>>> enabled by the user on that port, so hopefully for simple case (one >>>>> segment, no tx offloads) it >>>>> should be negligible. >>>>> Again, we can add manual check in testpmd tx-only code to decide do we >>>>> need a TX prepare >>>>> to be called or not. >>>>> Konstantin