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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

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?
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.