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

[Vladislav Zolotarov]

On Sep 11, 2015 5:55 PM, "Thomas Monjalon" <thomas.monjalon at 6wind.com>
wrote:
>
> 2015-09-11 17:47, Avi Kivity:
> > On 09/11/2015 05:25 PM, didier.pallard wrote:
> > > On 08/25/2015 08:52 PM, Vlad Zolotarov wrote:
> > >>
> > >> Helin, the issue has been seen on x540 devices. Pls., see a chapter
> > >> 7.2.1.1 of x540 devices spec:
> > >>
> > >> 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 (see
> > >> Section 7.2.3.3 for Tx Ring details and section Section 7.2.3.5.1 for
> > >> WTHRESH
> > >> details). For best performance it is recommended to minimize the
> > >> number of buffers
> > >> as possible.
> > >>
> > >> Could u, pls., clarify why do u think that the maximum number of data
> > >> buffers is limited by 8?
> > >>
> > >> thanks,
> > >> vlad
> > >
> > > 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.

>