On Wed, 28 Jun 2023 at 20:26, Július Milan <julius.milan.22@xxxxxxxxx> wrote:
>
> Hi all
>
> I am writing an AF_XDP based user space application.
> However in my use case, packets payload get fragmented while
> processing, basically new packets are constructed inside and sent
> further.
> I probably cannot avoid mempcy anyway.
>
> So I plan to solve it simply - one umem per port, no locking, no
> keeping track of umem frames presence (kernel / user space) . Just
> usage of the rings, one half of the frames to circulate between the RX
> <-> fill queue, the other half TX <-> completion queue.
>
> Is it actually possible to initialize the rings in such a way that at
> the very beginning I would fill the completion queue by some frames?
> This is to avoid multithreaded access to the free frames without
> locking (initial TX would take a look for free frames inside the
> completion queue).
You could fill the completion ring with entries from user space at
initialization time. As long as you always pick the first entry in the
completion ring before putting it on the Tx ring, the kernel would not
overwrite your entries. One complication is that you would have to
construct a new peek() routine for the completion queue, as the normal
one would indicate no entries found even though you have written
entries in it.
Maybe an easier idea is to just have some code like this:
/* Total N entries in Tx and completion ring.
* allocated initialized to 0 somewhere */
if (allocated < N) {
allocated++;
return N;
}
return next entry on completion queue;
Returns the buffer number in the umem and assumes your Tx buffers are
first in the umem. Though this scheme would introduce an if statement
in the path, it would be easier to start with. You need an address not
a number, and you likely also need some indication of "no buffer
available". Tried to keep it simple.
> If it's a bad idea, what else would you suggest?
>
> Thank you
> Julius
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