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 05603A0C41; Wed, 23 Jun 2021 16:56:50 +0200 (CEST) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 82DEF4003F; Wed, 23 Jun 2021 16:56:50 +0200 (CEST) Received: from mga07.intel.com (mga07.intel.com [134.134.136.100]) by mails.dpdk.org (Postfix) with ESMTP id 6E4DF4003E for ; Wed, 23 Jun 2021 16:56:48 +0200 (CEST) IronPort-SDR: T8CWL1EiYdW7X9jxTuJJxlUNNTTu5LP0fuLSUk1cywxcsfKrKGHPBLJQ5KpKf3kpb/W1Z0lruF 66niSEHdH4gA== X-IronPort-AV: E=McAfee;i="6200,9189,10024"; a="271122898" X-IronPort-AV: E=Sophos;i="5.83,294,1616482800"; d="scan'208";a="271122898" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Jun 2021 07:56:37 -0700 IronPort-SDR: H/f870U/bnnQ8BtuFJR5pCsEeLmgQcgnebpVgiFY1VFlw39A9gbfbaOCp6ecfO80LYeP65ydNM s+JpMlQOvyug== X-IronPort-AV: E=Sophos;i="5.83,294,1616482800"; d="scan'208";a="423720353" Received: from bricha3-mobl.ger.corp.intel.com ([10.252.13.79]) by orsmga002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-SHA; 23 Jun 2021 07:56:34 -0700 Date: Wed, 23 Jun 2021 15:56:30 +0100 From: Bruce Richardson To: fengchengwen Cc: Jerin Jacob , Thomas Monjalon , Ferruh Yigit , dpdk-dev , Nipun Gupta , Hemant Agrawal , Maxime Coquelin , Honnappa Nagarahalli , Jerin Jacob , David Marchand , Satananda Burla , Prasun Kapoor , Morten =?iso-8859-1?Q?Br=F8rup?= Message-ID: References: <1623763327-30987-1-git-send-email-fengchengwen@huawei.com> <25d29598-c26d-8497-2867-9b650c79df49@huawei.com> <3db2eda0-4490-2b8f-c65d-636bcf794494@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3db2eda0-4490-2b8f-c65d-636bcf794494@huawei.com> Subject: Re: [dpdk-dev] [RFC PATCH] dmadev: introduce DMA device library 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 Sender: "dev" This is developing into quite a long discussion with multiple threads ongoing at the same time. Since it's getting relatively hard to follow (at least for me), can I suggest that we actually hold a call to discuss "dmadev" and to move things along. Since most of the dicussion participants I believe are in the eastern timezones can I suggest 8AM UTC as a suitable timeslot (which would be 9AM Irish time, 1:30PM India and 4PM PRC time). Would 8AM UTC on Friday suit people? The usual tool for such community discussion is Jitsi (meet.jit.si/DPDK), so I would suggest re-using that for this discussion. Can anyone interested in participating in this discussion let me know [offlist, so we don't spam everyone], and I can try and co-ordinate if everyone is ok with above suggested timeslot and send out calendar invite. Regards, /Bruce On Wed, Jun 23, 2021 at 11:50:48AM +0800, fengchengwen wrote: > On 2021/6/23 1:51, Jerin Jacob wrote: > > On Fri, Jun 18, 2021 at 2:22 PM fengchengwen wrote: > >> > >> On 2021/6/17 22:18, Bruce Richardson wrote: > >>> On Thu, Jun 17, 2021 at 12:02:00PM +0100, Bruce Richardson wrote: > >>>> On Thu, Jun 17, 2021 at 05:48:05PM +0800, fengchengwen wrote: > >>>>> On 2021/6/17 1:31, Bruce Richardson wrote: > >>>>>> On Wed, Jun 16, 2021 at 05:41:45PM +0800, fengchengwen wrote: > >>>>>>> On 2021/6/16 0:38, Bruce Richardson wrote: > >>>>>>>> On Tue, Jun 15, 2021 at 09:22:07PM +0800, Chengwen Feng wrote: > >>>>>>>>> This patch introduces 'dmadevice' which is a generic type of DMA > >>>>>>>>> device. > >>>>>>>>> > >>>>>>>>> The APIs of dmadev library exposes some generic operations which can > >>>>>>>>> enable configuration and I/O with the DMA devices. > >>>>>>>>> > >>>>>>>>> Signed-off-by: Chengwen Feng > >>>>>>>>> --- > >>>>>>>> Thanks for sending this. > >>>>>>>> > >>>>>>>> Of most interest to me right now are the key data-plane APIs. While we are > >>>>>>>> still in the prototyping phase, below is a draft of what we are thinking > >>>>>>>> for the key enqueue/perform_ops/completed_ops APIs. > >>>>>>>> > >>>>>>>> Some key differences I note in below vs your original RFC: > >>>>>>>> * Use of void pointers rather than iova addresses. While using iova's makes > >>>>>>>> sense in the general case when using hardware, in that it can work with > >>>>>>>> both physical addresses and virtual addresses, if we change the APIs to use > >>>>>>>> void pointers instead it will still work for DPDK in VA mode, while at the > >>>>>>>> same time allow use of software fallbacks in error cases, and also a stub > >>>>>>>> driver than uses memcpy in the background. Finally, using iova's makes the > >>>>>>>> APIs a lot more awkward to use with anything but mbufs or similar buffers > >>>>>>>> where we already have a pre-computed physical address. > >>>>>>> > >>>>>>> The iova is an hint to application, and widely used in DPDK. > >>>>>>> If switch to void, how to pass the address (iova or just va ?) > >>>>>>> this may introduce implementation dependencies here. > >>>>>>> > >>>>>>> Or always pass the va, and the driver performs address translation, and this > >>>>>>> translation may cost too much cpu I think. > >>>>>>> > >>>>>> > >>>>>> On the latter point, about driver doing address translation I would agree. > >>>>>> However, we probably need more discussion about the use of iova vs just > >>>>>> virtual addresses. My thinking on this is that if we specify the API using > >>>>>> iovas it will severely hurt usability of the API, since it forces the user > >>>>>> to take more inefficient codepaths in a large number of cases. Given a > >>>>>> pointer to the middle of an mbuf, one cannot just pass that straight as an > >>>>>> iova but must instead do a translation into offset from mbuf pointer and > >>>>>> then readd the offset to the mbuf base address. > >>>>>> > >>>>>> My preference therefore is to require the use of an IOMMU when using a > >>>>>> dmadev, so that it can be a much closer analog of memcpy. Once an iommu is > >>>>>> present, DPDK will run in VA mode, allowing virtual addresses to our > >>>>>> hugepage memory to be sent directly to hardware. Also, when using > >>>>>> dmadevs on top of an in-kernel driver, that kernel driver may do all iommu > >>>>>> management for the app, removing further the restrictions on what memory > >>>>>> can be addressed by hardware. > >>>>> > >>>>> Some DMA devices many don't support IOMMU or IOMMU bypass default, so driver may > >>>>> should call rte_mem_virt2phy() do the address translate, but the rte_mem_virt2phy() > >>>>> cost too many CPU cycles. > >>>>> > >>>>> If the API defined as iova, it will work fine in: > >>>>> 1) If DMA don't support IOMMU or IOMMU bypass, then start application with > >>>>> --iova-mode=pa > >>>>> 2) If DMA support IOMMU, --iova-mode=pa/va work both fine > >>>>> > >>>> > >>>> I suppose if we keep the iova as the datatype, we can just cast "void *" > >>>> pointers to that in the case that virtual addresses can be used directly. I > >>>> believe your RFC included a capability query API - "uses void * as iova" > >>>> should probably be one of those capabilities, and that would resolve this. > >>>> If DPDK is in iova=va mode because of the presence of an iommu, all drivers > >>>> could report this capability too. > >>>> > >>>>>> > >>>>>>>> * Use of id values rather than user-provided handles. Allowing the user/app > >>>>>>>> to manage the amount of data stored per operation is a better solution, I > >>>>>>>> feel than proscribing a certain about of in-driver tracking. Some apps may > >>>>>>>> not care about anything other than a job being completed, while other apps > >>>>>>>> may have significant metadata to be tracked. Taking the user-context > >>>>>>>> handles out of the API also makes the driver code simpler. > >>>>>>> > >>>>>>> The user-provided handle was mainly used to simply application implementation, > >>>>>>> It provides the ability to quickly locate contexts. > >>>>>>> > >>>>>>> The "use of id values" seem like the dma_cookie of Linux DMA engine framework, > >>>>>>> user will get a unique dma_cookie after calling dmaengine_submit(), and then > >>>>>>> could use it to call dma_async_is_tx_complete() to get completion status. > >>>>>>> > >>>>>> > >>>>>> Yes, the idea of the id is the same - to locate contexts. The main > >>>>>> difference is that if we have the driver manage contexts or pointer to > >>>>>> contexts, as well as giving more work to the driver, it complicates the APIs > >>>>>> for measuring completions. If we use an ID-based approach, where the app > >>>>>> maintains its own ring of contexts (if any), it avoids the need to have an > >>>>>> "out" parameter array for returning those contexts, which needs to be > >>>>>> appropriately sized. Instead we can just report that all ids up to N are > >>>>>> completed. [This would be similar to your suggestion that N jobs be > >>>>>> reported as done, in that no contexts are provided, it's just that knowing > >>>>>> the ID of what is completed is generally more useful than the number (which > >>>>>> can be obviously got by subtracting the old value)] > >>>>>> > >>>>>> We are still working on prototyping all this, but would hope to have a > >>>>>> functional example of all this soon. > >>>>>> > >>>>>>> How about define the copy prototype as following: > >>>>>>> dma_cookie_t rte_dmadev_copy(uint16_t dev_id, xxx) > >>>>>>> while the dma_cookie_t is int32 and is monotonically increasing, when >=0 mean > >>>>>>> enqueue successful else fail. > >>>>>>> when complete the dmadev will return latest completed dma_cookie, and the > >>>>>>> application could use the dma_cookie to quick locate contexts. > >>>>>>> > >>>>>> > >>>>>> If I understand this correctly, I believe this is largely what I was > >>>>>> suggesting - just with the typedef for the type? In which case it obviously > >>>>>> looks good to me. > >>>>>> > >>>>>>>> * I've kept a single combined API for completions, which differs from the > >>>>>>>> separate error handling completion API you propose. I need to give the > >>>>>>>> two function approach a bit of thought, but likely both could work. If we > >>>>>>>> (likely) never expect failed ops, then the specifics of error handling > >>>>>>>> should not matter that much. > >>>>>>> > >>>>>>> The rte_ioat_completed_ops API is too complex, and consider some applications > >>>>>>> may never copy fail, so split them as two API. > >>>>>>> It's indeed not friendly to other scenarios that always require error handling. > >>>>>>> > >>>>>>> I prefer use completed operations number as return value other than the ID so > >>>>>>> that application could simple judge whether have new completed operations, and > >>>>>>> the new prototype: > >>>>>>> uint16_t rte_dmadev_completed(uint16_t dev_id, dma_cookie_t *cookie, uint32_t *status, uint16_t max_status, uint16_t *num_fails); > >>>>>>> > >>>>>>> 1) for normal case which never expect failed ops: > >>>>>>> just call: ret = rte_dmadev_completed(dev_id, &cookie, NULL, 0, NULL); > >>>>>>> 2) for other case: > >>>>>>> ret = rte_dmadev_completed(dev_id, &cookie, &status, max_status, &fails); > >>>>>>> at this point the fails <= ret <= max_status > >>>>>>> > >>>>>> Completely agree that we need to plan for the happy-day case where all is > >>>>>> passing. Looking at the prototypes you have above, I am ok with returning > >>>>>> number of completed ops as the return value with the final completed cookie > >>>>>> as an "out" parameter. > >>>>>> For handling errors, I'm ok with what you propose above, just with one > >>>>>> small adjustment - I would remove the restriction that ret <= max_status. > >>>>>> > >>>>>> In case of zero-failures, we can report as many ops succeeding as we like, > >>>>>> and even in case of failure, we can still report as many successful ops as > >>>>>> we like before we start filling in the status field. For example, if 32 ops > >>>>>> are completed, and the last one fails, we can just fill in one entry into > >>>>>> status, and return 32. Alternatively if the 4th last one fails we fill in 4 > >>>>>> entries and return 32. The only requirements would be: > >>>>>> * fails <= max_status > >>>>>> * fails <= ret > >>>>>> * cookie holds the id of the last entry in status. > >>>>> > >>>>> I think we understand the same: > >>>>> > >>>>> The fails <= ret <= max_status include following situation: > >>>>> 1) If max_status is 32, and there are 32 completed ops, then the ret will be 32 > >>>>> no matter which ops is failed > >>>>> 2) If max_status is 33, and there are 32 completed ops, then the ret will be 32 > >>>>> 3) If max_status is 16, and there are 32 completed ops, then the ret will be 16 > >>>>> > >>>>> and the cookie always hold the id of the last returned completed ops, no matter > >>>>> it's completed successful or failed > >>>>> > >>>> > >>>> I actually disagree on the #3. If max_status is 16, there are 32 completed > >>>> ops, and *no failures* the ret will be 32, not 16, because we are not > >>>> returning any status entries so max_status need not apply. Keeping that > >>>> same scenario #3, depending on the number of failures and the point of > >>>> them, the return value may similarly vary, for example: > >>>> * if job #28 fails, then ret could still be 32, cookie would be the cookie > >>>> for that job, "fails" parameter would return as 4, with status holding the > >>>> failure of 28 plus the succeeded status of jobs 29-31, i.e. 4 elements. > >>>> * if job #5 fails, then we can't fit the status list from 5 though 31 in an > >>>> array of 16, so "fails" == 16(max_status) and status contains the 16 > >>>> statuses starting from #5, which means that cookie contains the value for > >>>> job #20 and ret is 21. > >>>> > >>>> In other words, ignore max_status and status parameters *unless we have an > >>>> error to return*, meaning the fast-path/happy-day case works as fast as > >>>> possible. You don't need to worry about sizing your status array to be big, > >>>> and you always get back a large number of completions when available. Your > >>>> fastpath code only need check the "fails" parameter to see if status needs > >>>> to ever be consulted, and in normal case it doesn't. > >>>> > >>>> If this is too complicated, maybe we can simplify a little by returning just > >>>> one failure at a time, though at the cost of making error handling slower? > >>>> > >>>> rte_dmadev_completed(dev_id, &cookie, &failure_status) > >>>> > >>>> In this case, we always return the number of completed ops on success, > >>>> while on failure, we return the first error code. For a single error, this > >>>> works fine, but if we get a burst of errors together, things will work > >>>> slower - which may be acceptable if errors are very rare. However, for idxd > >>>> at least if a fence occurs after a failure all jobs in the batch after the > >>>> fence would be skipped, which would lead to the "burst of errors" case. > >>>> Therefore, I'd prefer to have the original suggestion allowing multiple > >>>> errors to be reported at a time. > >>>> > >>>> /Bruce > >>> > >>> Apologies for self-reply, but thinking about it more, a combination of > >>> normal-case and error-case APIs may be just simpler: > >>> > >>> int rte_dmadev_completed(dev_id, &cookie) > >>> > >>> returns number of items completed and cookie of last item. If there is an > >>> error, returns all successfull values up to the error entry and returns -1 > >>> on subsequent call. > >>> > >>> int rte_dmadev_completed_status(dev_id, &cookie, max_status, status_array, > >>> &error_count) > >>> > >>> this is a slower completion API which behaves like you originally said > >>> above, returning number of completions x, 0 <= x <= max_status, with x > >>> status values filled into array, and the number of unsuccessful values in > >>> the error_count value. > >>> > >>> This would allow code to be written in the application to use > >>> rte_dmadev_completed() in the normal case, and on getting a "-1" value, use > >>> rte_dmadev_completed_status() to get the error details. If strings of > >>> errors might be expected, the app can continually use the > >>> completed_status() function until error_count returns 0, and then switch > >>> back to the faster/simpler version. > >> > >> This two-function simplify the status_array's maintenance because we don't need init it to zero. > >> I think it's a good trade-off between performance and rich error info (status code). > >> > >> Here I'd like to discuss the 'burst size', which is widely used in DPDK application (e.g. > >> nic polling or ring en/dequeue). > >> Currently we don't define a max completed ops in rte_dmadev_completed() API, the return > >> value may greater than 'burst size' of application, this may result in the application need to > >> maintain (or remember) the return value of the function and special handling at the next poll. > >> > >> Also consider there may multiple calls rte_dmadev_completed to check fail, it may make it > >> difficult for the application to use. > >> > >> So I prefer following prototype: > >> uint16_t rte_dmadev_completed(uint16_t dev_id, dma_cookie_t *cookie, uint16_t nb_cpls, bool *has_error) > >> -- nb_cpls: indicate max process operations number > >> -- has_error: indicate if there is an error > >> -- return value: the number of successful completed operations. > >> -- example: > >> 1) If there are already 32 completed ops, and 4th is error, and nb_cpls is 32, then > >> the ret will be 3(because 1/2/3th is OK), and has_error will be true. > >> 2) If there are already 32 completed ops, and all successful completed, then the ret > >> will be min(32, nb_cpls), and has_error will be false. > >> 3) If there are already 32 completed ops, and all failed completed, then the ret will > >> be 0, and has_error will be true. > >> uint16_t rte_dmadev_completed_status(uint16_t dev_id, dma_cookie_t *cookie, uint16_t nb_status, uint32_t *status) > >> -- return value: the number of failed completed operations. > > > > > > > > In typical storage use cases etc, Sometimes application need to > > provide scatter-gather list, > > At least in our hardware sg list gives a "single completion result" > > and it stops on the first failure to restart > > the transfer by application. Have you thought of scatter-gather use > > case and how it is in other HW? > > cookie and request are in a one-to-one correspondence, whether the request is a single or sg-list. > Kunpeng9x0 don't support sg-list, I'm still investigating other hardware. > > The above 'restart the transfer by application' mean re-schedule request (and have one new cookie) or > just re-enable current failed request (this may introduce new API) ? > > > > > prototype like the following works for us: > > rte_dmadev_enq_sg(void **src, void **dest, unsigned int **length, int > > nb_segments, cookie, ,,,) > > OK, we could define one scatter-list struct to wrap src/dest/length. > > > > > > >> > >> The application use the following invocation order when polling: > >> has_error = false; // could be init to false by dmadev API, we need discuss > >> ret = rte_dmadev_completed(dev_id, &cookie, bust_size, &has_error); > >> // process successful completed case: > >> for (int i = 0; i < ret; i++) { > >> } > >> if (unlikely(has_error)) { > >> // process failed completed case > >> ret = rte_dmadev_completed_status(dev_id, &cookie, burst_size - ret, status_array); > >> for (int i = 0; i < ret; i++) { > >> // ... > >> } > >> } > >> > >> > >>> > >>> This two-function approach also allows future support for other DMA > >>> functions such as comparison, where a status value is always required. Any > >>> apps using that functionality would just always use the "_status" function > >>> for completions. > >>> > >>> /Bruce > >>> > >>> . > >>> > >> > > > > . > > >