Re: [dpdk-dev] Suggestions on how to customize the metadata fields of each packet

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Victor,

> 
> Thanks for your quick answer,
> 
> I have read so many documents and web pages on this issue that probably I
> confounded the utility of the headroom. It is good to know that this 128
> bytes space is available to my disposal. The fact of being lost once the
> NIC transmits the frame it is not a problem at all for my application.
> However, in case that this space is not enough, I have seen in the rte_mbuf
> struct a (void *) pointer called userdata which is in theory used for extra
> user-defined metadata. If I wanted to attach an additional metadata struct,
> I guess that I just have to assign the pointer to this struct to the
> userdata field. However, what happens if I want that the content of this
> struct travels with the packet through a software ring in order to be
> processed by another thread? Should I reserve more space in the ring to
> allocate such extra metadata?
> 
> Thanks again,


In theory headroom inside mbuf should be left for packet's data.
To do things properly you'll need to create your mbuf mempools with
priv_size >= your_extra_metadata_size.

Konstantin

> 
> PD: I have copied the message to users mailing list
> 
> 2018-02-23 4:13 GMT+01:00 <longtb5@viettel.com.vn>:
> 
> > Hi,
> >
> > First, I think your question should be sent to the user mailing list, not
> > the dev mailing list.
> >
> > > I have seen that each packet has a headroom memory space (128 bytes
> > long)
> >
> > > where RSS hashing and other metadata provided by the NIC is stored.
> >
> > If I’m not mistaken, the headroom is not where metadata provided by the
> > NIC are stored. Those metadata are stored in the rte_mbuf struct, which
> > is also 128 bytes long.
> >
> > The headroom area is located AFTER the end of rte_mbuf (at offset 128).
> > By default the headroom area is also 128 byte long, so the actual packet
> > data is stored at offset 256.
> >
> > You can store whatever you want in this headroom area. However those
> > information are lost as soon as the packet leaves DPDK (the NIC will start
> > sending at offset 256).
> >
> > -BL.
> >
> 
> 
> 
> --
> Victor