Re: AF_XDP socket (sock2) unable to read, after writing with another AF_XDP socket (sock1)

[Konrad Djimeli <kdjimeli@igalia.com>]

On 2018-11-10 15:58, William Tu wrote:
> On Sat, Nov 10, 2018 at 6:31 AM Björn Töpel <bjorn.topel@gmail.com> wrote:
>>
>> Den tors 8 nov. 2018 kl 16:15 skrev Konrad Djimeli <kdjimeli@igalia.com>:
>> >
>> [...]
>> > > In terms of correctness:
>> > >
>> > > In 3. you pass the same UMEM indicies to the Tx ring (ids={0,
>> > > FRAME_SIZE, 2*FRAME_SIZE, ...}), as you have passed to the fill
>> > > ring. This is not correct. Userspace to track what ids are owned by
>> > > the user and by the kernel. Now, both the fill ring and Tx ring
>> > > contain the same id.
>> > Please I have been looking into this for some days now, but I have not
>> > been able to understand what you where referring to and how to fix it.
>> >
>>
>> Ok, let's talk about the basics, and hopefully it will clear up. The
>> indicies/ids into the UMEM are used to point out packet buffers.
>>
>> The Rx/Tx/Completion/Fill rings are used to pass ownership between the
>> kernel and the user application. E.g. if fill id X with data and pass
>> that to the kernel for transmission, the application has passed
>> ownership of buffer X to the kernel. The application cannot (well,
>> shouldn't) touch the buffer pointed out by X until the kernel is done
>> with the buffer -- in other words, when the buffer is passed back to
>> the application via the completion ring. Analogous, when you pass id Y
>> to the fill ring, Y is owned by the kernel. The application cannot
>> (again, shouldn't) touch the data pointed out by Y until ownership is
>> passed back to the application via the Rx ring.
>>
> 
> Hi Konrad,
> 
> I made a slide about using how the rx/tx/completion/fill ring works for OVS.
> Hope this is helpful for you.
> https://drive.google.com/open?id=1gM3lndS6WeS16QNk74L22UNx5D5wlS2o
> 
> Another thing for debugging is first to add a print in the XDP code,
> so making sure the packet is there at the XDP layer.
> https://github.com/williamtu/ovs-ebpf/blob/afxdp-array/bpf/xdp.h#L71
> 
> Then I usually prints the xsk stats
> https://github.com/williamtu/ovs-ebpf/blob/afxdp-array/lib/netdev-linux.c#L656
> So making sure I'm not mess up with these rings.
> 
> Regards,
> William

Hello William,

Thanks a lot. I have gone through the resources you provided, and they
have been 
very helpful, especially the document on how the rx/tx/completion/fill
ring works.

Thanks,
Konrad