RE: min_mbuf_head/tailroom_req in rte_cryptodev_info

[Suanming Mou <suanmingm@nvidia.com>]

Hi Anoob,

Thanks for the quick response. Nice answering. I think I have fully understood the fields meaning.

Many thanks again,
Suanming

> -----Original Message-----
> From: Anoob Joseph <anoobj@marvell.com>
> Sent: Tuesday, May 9, 2023 8:13 PM
> To: Suanming Mou <suanmingm@nvidia.com>
> Cc: Akhil Goyal <gakhil@marvell.com>; dev@dpdk.org; Doherty, Declan
> <declan.doherty@intel.com>
> Subject: RE: min_mbuf_head/tailroom_req in rte_cryptodev_info
> 
> Hi Suanming,
> 
> Please see inline.
> 
> Thanks,
> Anoob
> 
> > From: Suanming Mou <suanmingm@nvidia.com>
> > Sent: Tuesday, May 9, 2023 5:23 PM
> > To: Anoob Joseph <anoobj@marvell.com>; Doherty, Declan
> <declan.doherty@intel.com>
> > Cc: Akhil Goyal <gakhil@marvell.com>; dev@dpdk.org
> > Subject: [EXT] min_mbuf_head/tailroom_req in rte_cryptodev_info
> >
> > External Email
> > ________________________________________
> > Hi Guys,
> >
> > Sorry for disturbing you. I have one small question regarding the
> min_mbuf_head/tailroom_req in rte_cryptodev_info.
> > I saw you were the people who discussed, reviewed and added that two fields.
> > IIUC, the two fields provided a hint to the PMD that HW can use(modify) the
> headroom and tailroom space to improve the performance.
> [Anoob] That's right.
> 
> > But regarding the space(e.g. headroom) user reserved, should the data
> offset(sym_op->aead.data.offset) in struct rte_crypto_op also be taken into
> account?
> > e.g. if the mbuf in the operation is something like below:
> > |---headroom--|----op offset---|---payload to process---|---tailroom---|
> 
> [Anoob] Above layout is correct. Headroom that can be used by cryptodev is the
> headroom of the mbuf.
> 
> > In that case, what is the headroom mean to HW? Start from real mbuf
> headroom before op offset or the offset before payload?
> 
> [Anoob] May be let me explain how we use it in CNXK PMDs. We have to pass
> fields such as IV & offsets to the hardware for performing crypto operation. To
> maximize performance, we pass it as single buffer and hence the requirement on
> this headroom.
> 
> In our enqueue path, we use headroom for storing some of these transient data.
> 
> |---headroom--|-------------- packet_len ---------------------|---tailroom---|
> |---headroom--|----op offset---|---payload to process---|---tailroom---|
>                |--------|   <---- Space used by cryptodev for internal purpose
>                             ^ = rte_pktmbuf_mtod(mbuf, void *);
> 
> All offsets mentioned in rte_crypto_op would be calculated from the packet
> start [rte_pktmbuf_mtod()]. In other words, the packet data from the start of
> the packet till offset would be untouched by the cryptodev in all circumstances.
> It is the space that is before start of the packet(ie headroom of mbuf) which
> would be used for this purpose.
> 
> Hope it is clear now.