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

["Richardson, Bruce" <bruce.richardson@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Vladislav Zolotarov
> Sent: Friday, September 11, 2015 4:13 PM
> To: Thomas Monjalon
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1
> for all NICs but 82598
> 
> On Sep 11, 2015 5:55 PM, "Thomas Monjalon" <thomas.monjalon@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.
> 
Actually, it could very well do - we'd have to test it. For the vector IO
paths, every additional cycle in the RX or TX paths causes a noticeable perf
drop.

/Bruce