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 6:43 PM, "Avi Kivity" <avi@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@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.