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

[Vladislav Zolotarov <vladz@cloudius-systems.com>]

On Sep 11, 2015 7:09 PM, "Thomas Monjalon" <thomas.monjalon@6wind.com>
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?
> 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.

We thought about linearization too. It's doable with extra mempool and it
may be optional so that those that don't need could compile it out and/or
disable it in a runtime...