[dpdk-dev] i40e xmit path HW limitation

[Vlad Zolotarov]

On 07/30/15 20:33, Zhang, Helin wrote:
>
>> -----Original Message-----
>> From: Vlad Zolotarov [mailto:vladz at cloudius-systems.com]
>> Sent: Thursday, July 30, 2015 9:44 AM
>> To: Zhang, Helin; Ananyev, Konstantin
>> Cc: dev at dpdk.org
>> Subject: Re: i40e xmit path HW limitation
>>
>>
>>
>> On 07/30/15 19:10, Zhang, Helin wrote:
>>>> -----Original Message-----
>>>> From: Vlad Zolotarov [mailto:vladz at cloudius-systems.com]
>>>> Sent: Thursday, July 30, 2015 7:58 AM
>>>> To: dev at dpdk.org; Ananyev, Konstantin; Zhang, Helin
>>>> Subject: RFC: i40e xmit path HW limitation
>>>>
>>>> Hi, Konstantin, Helin,
>>>> there is a documented limitation of xl710 controllers (i40e driver)
>>>> which is not handled in any way by a DPDK driver.
>>>>    From the datasheet chapter 8.4.1:
>>>>
>>>> "• A single transmit packet may span up to 8 buffers (up to 8 data
>>>> descriptors per packet including both the header and payload buffers).
>>>> • The total number of data descriptors for the whole TSO (explained
>>>> later on in this chapter) is unlimited as long as each segment within
>>>> the TSO obeys the previous rule (up to 8 data descriptors per segment
>>>> for both the TSO header and the segment payload buffers)."
>>> Yes, I remember the RX side just supports 5 segments per packet receiving.
>>> But what's the possible issue you thought about?
>> Note that it's a Tx size we are talking about.
>>
>> See 30520831f058cd9d75c0f6b360bc5c5ae49b5f27 commit in linux net-next repo.
>> If such a cluster arrives and you post it on the HW ring - HW will shut this HW ring
>> down permanently. The application will see that it's ring is stuck.
> That issue was because of using more than 8 descriptors for a packet for TSO.

There is no problem in transmitting the TSO packet with more than 8 
fragments.
On the opposite - one can't transmit a non-TSO packet with more than 8 
fragments.
One also can't transmit the TSO packet that would contain more than 8 
fragments in a single TSO segment including the TSO headers.

Pls., read the HW spec as I quoted above for more details.

>
>>>> This means that, for instance, long cluster with small fragments has to be
>>>> linearized before it may be placed on the HW ring.
>>> What type of size of the small fragments? Basically 2KB is the default size of
>> mbuf of most
>>> example applications. 2KB x 8 is bigger than 1.5KB. So it is enough for the
>> maximum
>>> packet size we supported.
>>> If 1KB mbuf is used, don't expect it can transmit more than 8KB size of packet.
>> I kinda lost u here. Again, we talk about the Tx side here and buffers
>> are not obligatory completely filled. Namely there may be a cluster with
>> 15 fragments 100 bytes each.
> The root cause is using more than 8 descriptors for a packet.

That would be if u would like to SUPER simplify the HW limitation above. 
In that case u would significantly limit the different packets that may 
be sent without the linearization.

> Linux driver can help
> on reducing number of descriptors to be used by merging small size of payload
> together, right?
> It is not for TSO, it is just for packet transmitting. 2 options in my mind:
> 1. Use should ensure it will not use more than 8 descriptors per packet for transmitting.

This requirement is too restricting. Pls., see above.

> 2. DPDK driver should try to merge small packet together for such case, like Linux kernel driver.
> I prefer to use option 1, users should ensure that in the application or up layer software,
> and keep the PMD driver as simple as possible.

The above statement is super confusing: on the one hand u suggest the 
DPDK driver to merge the small packet (fragments?) together (how?) and 
then u immediately propose the user application to do that. Could u, 
pls., clarify what exactly u suggest here?
If that's to leave it to the application - note that it would demand 
patching all existing DPDK applications that send TCP packets.

>
> But I have a thought that the maximum number of RX/TX descriptor should be able to be
> queried somewhere.

There is no such thing as maximum number of Tx fragments in a TSO case. 
It's only limited by the Tx ring size.

>
> Regards,
> Helin
>>>> In more standard environments like Linux or FreeBSD drivers the solution is
>>>> straight forward - call skb_linearize()/m_collapse() corresponding.
>>>> In the non-conformist environment like DPDK life is not that easy - there is no
>>>> easy way to collapse the cluster into a linear buffer from inside the device
>> driver
>>>> since device driver doesn't allocate memory in a fast path and utilizes the user
>>>> allocated pools only.
>>>> Here are two proposals for a solution:
>>>>
>>>>    1. We may provide a callback that would return a user TRUE if a give
>>>>       cluster has to be linearized and it should always be called before
>>>>       rte_eth_tx_burst(). Alternatively it may be called from inside the
>>>>       rte_eth_tx_burst() and rte_eth_tx_burst() is changed to return some
>>>>       error code for a case when one of the clusters it's given has to be
>>>>       linearized.
>>>>    2. Another option is to allocate a mempool in the driver with the
>>>>       elements consuming a single page each (standard 2KB buffers would
>>>>       do). Number of elements in the pool should be as Tx ring length
>>>>       multiplied by "64KB/(linear data length of the buffer in the pool
>>>>       above)". Here I use 64KB as a maximum packet length and not taking
>>>>       into an account esoteric things like "Giant" TSO mentioned in the
>>>>       spec above. Then we may actually go and linearize the cluster if
>>>>       needed on top of the buffers from the pool above, post the buffer
>>>>       from the mempool above on the HW ring, link the original cluster to
>>>>       that new cluster (using the private data) and release it when the
>>>>       send is done.
>>>>
>>>>
>>>> The first is a change in the API and would require from the application some
>>>> additional handling (linearization). The second would require some additional
>>>> memory but would keep all dirty details inside the driver and would leave the
>>>> rest of the code intact.
>>>>
>>>> Pls., comment.
>>>>
>>>> thanks,
>>>> vlad
>>>>