Relationship between Rx/Tx ring and Skbuff.

Relationship between Rx/Tx ring and Skbuff.

[Vasu M]

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Hi,

I would like to understand the journey of the packet in Linux kernel. There
are many resources that explain this differently but as I have understood
it on a high level, when a packet is received in a NIC, DMA  moves the
packets from the NIC frame buffer into the RX ring buffer in the driver. A
hardware interrupt is then raised and the top half moves the packet to the
RX ring buffer. Is it right to understand that the bottom half tasklets
then drain the buffer and move the packets to sk_buff data structure? Are
the RX/TX buffers different from sk_buff in terms of memory regions or the
sk_buff simply points to the physical address of the packets residing in
the RX ring and then does the manipulation of the pointers to send it up
in the kernel IP stack?

I know this is a broad topic but I would like to seek clarification from
the experts on my understanding of packet movement inside the kernel.  Or
can you point me to a resource which can be a close explanation about the
implementation on how the kernel handles in > 4.x ?

Thanks,
Vasu

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Re: Relationship between Rx/Tx ring and Skbuff.

[Valdis Klētnieks]

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On Wed, 05 Aug 2020 11:34:52 -0700, Vasu M said:

> it on a high level, when a packet is received in a NIC, DMA  moves the
> packets from the NIC frame buffer into the RX ring buffer in the driver. A
> hardware interrupt is then raised and the top half moves the packet to the
> RX ring buffer.

Umm... I would expect the packet to be moved to the RX ring buffer exactly once,
not twice...

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Re: Relationship between Rx/Tx ring and Skbuff.

[Vasu M]

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> On Wed, 05 Aug 2020 11:34:52 -0700, Vasu M said:
>
> > it on a high level, when a packet is received in a NIC, DMA  moves the
> > packets from the NIC frame buffer into the RX ring buffer in the driver.
> A
> > hardware interrupt is then raised and the top half moves the packet to
> the
> > RX ring buffer.
>
> Umm... I would expect the packet to be moved to the RX ring buffer exactly
> once,
> not twice...
>

I am sorry, yes, thats a mistake and it makes sense. If the DMA moves the
packet to the RX ring, the top half just acknowledges the interrupt and
schedules the softirq to drain the packets from RX. Subsequently, after the
packets go through sk_buff, they get queued in the socket recv buffer and
get to user space memory finally? I suppose I am only scratching the
surface leaving so many details.

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Re: Relationship between Rx/Tx ring and Skbuff.

[Richard Sailer]

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Hi,

Fortunately I wrote a seminar paper some years ago at university about
exactly that topic. It might help and should still be up to date in all
important regards. https://github.com/richi235/ftrace-paper

I used ftrace to trace the way of a packet, the pdf is in Document/

Cheers,
-- Richard


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Re: Relationship between Rx/Tx ring and Skbuff.

[Mulyadi Santosa]

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On Fri, Aug 28, 2020 at 8:44 PM Richard Sailer <
richard_siegfried@systemli.org> wrote:

> Hi,
>
> Fortunately I wrote a seminar paper some years ago at university about
> exactly that topic. It might help and should still be up to date in all
> important regards. https://github.com/richi235/ftrace-paper
>
> I used ftrace to trace the way of a packet, the pdf is in Document/
>
> Cheers,
> -- Richard
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies@kernelnewbies.org
> https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>

Great work!!!!!
-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

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