Re: [lwip-devel] in-place overwriting of payload via static "tcphdr" pointer.

[Bernhart Pelger]

Am 08.01.2018 um 20:56 schrieb address@hidden:
> Bernhart Pelger wrote:
> > [..]
> > The Xilinx driver does not use custom_pbufs (as suggested in 
> > ZeroCopyRx.c).
> > Therefore it cannot know when a pbuf is not needed any more.
> > The Xilinx driver immediately frees the DMA buffer at RX interrupt (yes,
> > before LWIP has processed it).
> >
> > I know this sounds fundamentally broken, because other arriving packets
> > might be overwriting the free'd DMA buffers before LWIP or the
> > application had a chance to process it.
>
> This does not *sound* fundamentally broken, it *is*!
But that's a different topic, not in the scope of this topic, or even 
this mailing list.
And it doesn't affect me right now with my usage pattern.

> What do you mean by "this (byte-flipping?) can be delayed"?
> > Do you mean "this (byte-flipping) can cause a delay"?
>
> I meant because of handling out-of-sequence packets at a later point, the
> header fields are touched again. Then, they have to be converted again.
And that is a problem because ...?...
Worried about perfromance impact from the conversion operation?
Byte-flipping should be pretty fast (at least compared to memory 
accessses and conditional statements).

Also, packets arriving in-order is the more likely situation. It's not 
worth optimizing for that special
case where packets are arriving in wrong order.


> [..]
> > Ok, leaving the application programmer more freedom is fine.
> > But what about the freedom for the driver programmer, to use single
> > cache-invalidates for some performance gain?
>
> That's what I meant. If someone would make a patch changin TCP to not
> write back into the pbuf, I'd probably push it. I just don't want to do
> the work ;-)
I might do it at some point, but right now I have no time and
a lack of motivation, since invalidating at tcp_input() works for me
and is fast enough: only one 32-byte cache-line for the tcp header to 
invalidate.

Regards, Bernhart.