> -----Original Message----- > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Vladislav Zolotarov > Sent: Friday, September 11, 2015 5:04 PM > To: Avi Kivity > 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 6:43 PM, "Avi Kivity" <avi at cloudius-systems.com> wrote: > > > > On 09/11/2015 06:12 PM, Vladislav Zolotarov wrote: > >> > >> > >> 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. > >> > >> > > > > 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. > > An appropriate statistics counter would be a perfect tool to detect such > issues. Knowingly sending a packet that will cause a HW to hang is not > acceptable.
I would agree. Drivers should provide a function to query the max number of segments they can accept and the driver should be able to discard any packets exceeding that number, and just track it via a stat. /Bruce