Re: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for all NICs but 82598

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Avi Kivity
> Sent: Friday, September 11, 2015 6:48 PM
> To: Thomas Monjalon; Vladislav Zolotarov; didier.pallard
> Cc: dev@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@6wind.com
> >>> <mailto:thomas.monjalon@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