Re: [dpdk-dev] PMD/l3fwd issue

[Harish Patil <harish.patil@qlogic.com>]

Hi Konstantin,

>Hi Patil,
>
>> -----Original Message-----
>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Harish Patil
>> Sent: Thursday, September 03, 2015 4:53 PM
>> To: dev@dpdk.org
>> Subject: [dpdk-dev] PMD/l3fwd issue
>>
>> Hello,
>> Have a question regarding l3fwd application. The l3fwd application
>>expects
>> the poll mode driver to return packets whose L2 header is 16-byte
>>aligned.
>
>Yep, and as I remember, by default PMD returns ti the upper layer mbufs
>with data offsets
>aligned to cahce line size (64B).
>Unless you'll change RTE_PKTMBUF_HEADROOM config parameter.
>
>> Otherwise, it results in a crash. This is due to use of _mm_load_si128()
>> and _mm_store_si128() intrinsics which expects the address to be 16-byte
>> aligned. However, most of the real protocol stack expects packets such
>> that its IP header be aligned on a 16-byte boundary (not L2). Its not
>>just
>> for IP but any L3 for that matter.  That’s way we usually see
>> skb_reserve(skb, NET_IP_ALIGN) calls in linux drivers.
>
>Well, l3fwd is just an example application to demonstrate usage of DPDK
>API
>And max performance it could get for that type of workload.
>No-one forces you to use aligned load/store in your own application.

Yes, I agree if its our private application. But l3fwd being widely used
as a benchmarking/testing tool and they may ran into this issue.

>
>>
>> So I’m looking for suggestions here, whether l3wd application or poll
>>mode
>> driver should be changed to fix that? What is the right thing to do?
>> Can a check be added in l3fwd to use _mm_loadu_si128/_mm_storeu_si128
>> instructions instead of mm_load_si128/_mm_store_si128 if address is
>>found
>> not be 16B aligned?
>
>I'd personally just change l3fwd to use to use
>_mm_loadu_si128/_mm_storeu_si128 unconditionally.
>As by default  address is 16B aligned anyway, I think that using MOVDQU
>instead of MOVDQA here
>shouldn't make that big difference.
>But off course testing need to be done to make sure there is no
>performance drop with that change.

I too would just change l3fwd application so that all poll mode drivers
would just work. Are you proposing that we upstream l3fwd change if we
don’t see performance drop?

>Konstantin
>
>>
>> Thanks,
>> Harish
>>
>>
>>
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