Getting mbuf pointer from buf_addr pointer

Getting mbuf pointer from buf_addr pointer

[Antonio Di Bacco]

Is there any API that allows me to get the MBUF pointer given its
buf_addr pointer?

Regards,
Antonio.

Re: Getting mbuf pointer from buf_addr pointer

[Dmitry Kozlyuk]

2022-07-11 11:21 (UTC+0200), Antonio Di Bacco:
> Is there any API that allows me to get the MBUF pointer given its
> buf_addr pointer?

No.

How did you come across this need?
Sounds like an app design flaw.
What is the original problem you're solving?

P.S. It may be possible in specific cases when you know (from code)
how mempool sets buf_addr for a given mbuf,
and that PMDs don't change buf_addr with the used set of offloads.
Apparently, this approach would be extremely brittle and dangerous.

Re: Getting mbuf pointer from buf_addr pointer

[Antonio Di Bacco]

I was asked to reproduce the exact same behaviour of a library that
used VMDQ queues without DPDK.
Now this library offered an API to allocate a buffer for a packet and
returned an address where the application could write its data.

void* vmdqNew (vmdqUidPOOLt* p);

I implemented the same API to allocate an MBUF from a pool and I also
return the right address as a void* but then to transmit the packet I
need the address of the MBUF for the tx_burst API and I don't have it.

On Tue, Jul 12, 2022 at 10:32 AM Dmitry Kozlyuk
<dmitry.kozliuk@gmail.com> wrote:
>
> 2022-07-11 11:21 (UTC+0200), Antonio Di Bacco:
> > Is there any API that allows me to get the MBUF pointer given its
> > buf_addr pointer?
>
> No.
>
> How did you come across this need?
> Sounds like an app design flaw.
> What is the original problem you're solving?
>
> P.S. It may be possible in specific cases when you know (from code)
> how mempool sets buf_addr for a given mbuf,
> and that PMDs don't change buf_addr with the used set of offloads.
> Apparently, this approach would be extremely brittle and dangerous.

Re: Getting mbuf pointer from buf_addr pointer

[Dmitry Kozlyuk]

2022-07-12 12:30 (UTC+0200), Antonio Di Bacco:
> I was asked to reproduce the exact same behaviour of a library that
> used VMDQ queues without DPDK.
> Now this library offered an API to allocate a buffer for a packet and
> returned an address where the application could write its data.
> 
> void* vmdqNew (vmdqUidPOOLt* p);
> 
> I implemented the same API to allocate an MBUF from a pool and I also
> return the right address as a void* but then to transmit the packet I
> need the address of the MBUF for the tx_burst API and I don't have it.

Since you create mbufs and send them, the approach I mentioned is viable.
See rte_mbuf_buf_addr() and do the reverse to calculate the pointer to mbuf.
There is no API because it cannot work for any mbuf, but you case is limited
(there are chained mbufs, externally-attached mbufs, etc).
BTW, in general you could save a back-reference to the mbuf
just before the buffer you return from vmdqNew(), in pktmbuf private data.
But direct calculation is more efficient.