On 09/13/2015 02:47 PM, Ananyev, Konstantin wrote:
>
>> -----Original Message-----
>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Avi Kivity
>> Sent: Friday, September 11, 2015 6:48 PM
>> To: Thomas Monjalon; Vladislav Zolotarov; didier.pallard
>> 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 09/11/2015 07:08 PM, Thomas Monjalon wrote:
>>> 2015-09-11 18:43, Avi Kivity:
>>>> On 09/11/2015 06:12 PM, Vladislav Zolotarov wrote:
>>>>> On Sep 11, 2015 5:55 PM, "Thomas Monjalon" <thomas.monjalon at 6wind.com
>>>>> <mailto:thomas.monjalon at 6wind.com>> wrote:
>>>>>> 2015-09-11 17:47, Avi Kivity:
>>>>>>> On 09/11/2015 05:25 PM, didier.pallard wrote:
>>>>>>>> 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.
>>> I have the feeling that everybody agrees on the same thing:
>>> the application must be able to make a well formed packet by checking
>>> limitations of the port. What about a field rte_eth_dev_info.max_tx_segs?
>> It is not generic enough. i40e has a limit that it imposes post-TSO.
>>
>>
>>> In case the application fails in its checks, the driver must drop it and
>>> notify the user via a stat counter.
>>> The driver can also remove the hardware limitation by gathering the segments
>>> but it may be hard to implement and would be a slow operation.
>> I think that to satisfy both the 64b full line rate applications and the
>> more complicated full stack applications, this must be made optional.
>> In particular, and application that only forwards packets will never hit
>> a NIC's limits, so it need not take any action. That's why I think a
>> verification function is ideal; a forwarding application can ignore it,
>> and a complex application can call it, and if it fails the packet, it
>> can linearize it itself, removing complexity from dpdk itself.
> I think that's a good approach to that problem.
> As I remember we discussed something similar a while ago -
> A function (tx_prep() or something) that would check nb_segs and probably
> some other HW specific restrictions,
> calculate pseudo-header checksum, reset ip header len, etc.
>
> From other hand we also can add two more fields into rte_eth_dev_info:
> 1) Max num of segs per TSO packet (tx_max_seg ?).
> 2) Max num of segs per single packet/TSO segment (tx_max_mtu_seg ?).
> So for ixgbe both will have value 40 - wthresh,
> while for i40e 1) would be UINT8_MAX and 2) will be 8.
> Then upper layer can use that information to select an optimal size for its
> TX buffers.
>
>
This will break whenever the fevered imagination of hardware designers
comes up with a new limit.
We can have an internal function that accepts these two parameters, and
then the driver-specific function can call this internal function:
static bool i40e_validate_packet(mbuf* m) {
return rte_generic_validate_packet(m, 0, 8);
}
static bool ixgbe_validate_packet(mbuf* m) {
return rte_generic_validate_packet(m, 40, 2);
}
this way, the application is isolated from changes in how invalid
packets are detected.
