> -----Original Message----- > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Vladislav Zolotarov > Sent: Friday, September 11, 2015 4:13 PM > To: Thomas Monjalon > Cc: dev at 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 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. > 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