> -----Original Message----- > From: Avi Kivity [mailto:avi at cloudius-systems.com] > Sent: Sunday, September 13, 2015 1:33 PM > To: Ananyev, Konstantin; 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 09/13/2015 02:47 PM, Ananyev, Konstantin wrote: > > > >> -----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. > > > > > > This will break whenever the fevered imagination of hardware designers > comes up with a new limit. > > We can have an internal function that accepts these two parameters, and > then the driver-specific function can call this internal function: > > static bool i40e_validate_packet(mbuf* m) { > return rte_generic_validate_packet(m, 0, 8); > } > > static bool ixgbe_validate_packet(mbuf* m) { > return rte_generic_validate_packet(m, 40, 2); > } > > this way, the application is isolated from changes in how invalid > packets are detected. > >
I am not saying we shouldn't have tx_prep (tx_validate?) function per PMD. As I said before I like that approach. I think we should have tx_prep (as you suggested) that most people using full-path TX would call, *plus* these extra fields in re_eth_dev_conf, so if someone needs that information - it would be there. Konstantin
