> -----Original Message----- > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Avi Kivity > Sent: Friday, September 11, 2015 6:48 PM > To: Thomas Monjalon; Vladislav Zolotarov; didier.pallard > Cc: dev at dpdk.org > Subject: Re: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for > all NICs but 82598 > > On 09/11/2015 07:08 PM, Thomas Monjalon wrote: > > 2015-09-11 18:43, Avi Kivity: > >> On 09/11/2015 06:12 PM, Vladislav Zolotarov wrote: > >>> On Sep 11, 2015 5:55 PM, "Thomas Monjalon" <thomas.monjalon at 6wind.com > >>> <mailto:thomas.monjalon at 6wind.com>> wrote: > >>>> 2015-09-11 17:47, Avi Kivity: > >>>>> On 09/11/2015 05:25 PM, didier.pallard wrote: > >>>>>> Hi vlad, > >>>>>> > >>>>>> Documentation states that a packet (or multiple packets in transmit > >>>>>> segmentation) can span any number of > >>>>>> buffers (and their descriptors) up to a limit of 40 minus WTHRESH > >>>>>> minus 2. > >>>>>> > >>>>>> Shouldn't there be a test in transmit function that drops > >>> properly the > >>>>>> mbufs with a too large number of > >>>>>> segments, while incrementing a statistic; otherwise transmit > >>> function > >>>>>> may be locked by the faulty packet without > >>>>>> notification. > >>>>>> > >>>>> What we proposed is that the pmd expose to dpdk, and dpdk expose > >>> to the > >>>>> application, an mbuf check function. This way applications that can > >>>>> generate complex packets can verify that the device will be able to > >>>>> process them, and applications that only generate simple mbufs can > >>> avoid > >>>>> the overhead by not calling the function. > >>>> More than a check, it should be exposed as a capability of the port. > >>>> Anyway, if the application sends too much segments, the driver must > >>>> drop it to avoid hang, and maintain a dedicated statistic counter to > >>>> allow easy debugging. > >>> I agree with Thomas - this should not be optional. Malformed packets > >>> should be dropped. In the icgbe case it's a very simple test - it's a > >>> single branch per packet so i doubt that it could impose any > >>> measurable performance degradation. > >> A drop allows the application no chance to recover. The driver must > >> either provide the ability for the application to know that it cannot > >> accept the packet, or it must fix it up itself. > > I have the feeling that everybody agrees on the same thing: > > the application must be able to make a well formed packet by checking > > limitations of the port. What about a field rte_eth_dev_info.max_tx_segs? > > It is not generic enough. i40e has a limit that it imposes post-TSO. > > > > In case the application fails in its checks, the driver must drop it and > > notify the user via a stat counter. > > The driver can also remove the hardware limitation by gathering the segments > > but it may be hard to implement and would be a slow operation. > > I think that to satisfy both the 64b full line rate applications and the > more complicated full stack applications, this must be made optional. > In particular, and application that only forwards packets will never hit > a NIC's limits, so it need not take any action. That's why I think a > verification function is ideal; a forwarding application can ignore it, > and a complex application can call it, and if it fails the packet, it > can linearize it itself, removing complexity from dpdk itself.
I think that's a good approach to that problem. As I remember we discussed something similar a while ago - A function (tx_prep() or something) that would check nb_segs and probably some other HW specific restrictions, calculate pseudo-header checksum, reset ip header len, etc. >From other hand we also can add two more fields into rte_eth_dev_info: 1) Max num of segs per TSO packet (tx_max_seg ?). 2) Max num of segs per single packet/TSO segment (tx_max_mtu_seg ?). So for ixgbe both will have value 40 - wthresh, while for i40e 1) would be UINT8_MAX and 2) will be 8. Then upper layer can use that information to select an optimal size for its TX buffers. Konstantin
