[dpdk-dev] [EXT] Re: [PATCH 1/3] mbuf: add Tx offloads for packet marking

[Nithin Dabilpuram]

On Wed, Jun 03, 2020 at 10:28:44AM +0200, Olivier Matz wrote:
> Hi,
> 
> On Tue, Jun 02, 2020 at 07:55:37PM +0530, Nithin Dabilpuram wrote:
> > On Tue, Jun 02, 2020 at 10:53:08AM +0000, Ananyev, Konstantin wrote:
> > > Hi Jerin,
> > > 
> > > > > > > I also share Olivier's concern about consuming 3 bits in ol_flags for that feature.
> > > > > > > Can it probably be squeezed somehow?
> > > > > > > Let say we reserve one flag that this information is present or not, and
> > > > > > > re-use one of rx-only fields for store additional information (packet_type, or so).
> > > > > > > Or might be some other approach.
> > > > > >
> > > > > > We are fine with this approach where we define one bit in Tx offloads for pkt
> > > > > > marking and and 3 bits reused from Rx offload flags area.
> > > > > >
> > > > > > For example:
> > > > > >
> > > > > > @@ -186,10 +186,16 @@ extern "C" {
> > > > > >
> > > > > >  /* add new RX flags here, don't forget to update PKT_FIRST_FREE */
> > > > > >
> > > > > > +/* Reused Rx offload bits for Tx offloads */
> > > > > > +#define PKT_X_TX_MARK_VLAN_DEI         (1ULL << 0)
> > > > > > +#define PKT_X_TX_MARK_IP_DSCP          (1ULL << 1)
> > > > > > +#define PKT_X_TX_MARK_IP_ECN           (1ULL << 2)
> > > > > > +
> > > > > >  #define PKT_FIRST_FREE (1ULL << 23)
> > > > > > -#define PKT_LAST_FREE (1ULL << 40)
> > > > > > +#define PKT_LAST_FREE (1ULL << 39)
> > > > > >
> > > > > >  /* add new TX flags here, don't forget to update PKT_LAST_FREE  */
> > > > > > +#define PKT_TX_MARK_EN         (1ULL << 40)
> > > > > >
> > > > > > Is this fine ?
> > > > >
> > > > > Any thoughts on this approach which uses only 1 bit in Tx flags out of 18
> > > > > and reuse unused Rx flag bits ?
> > > 
> > > My thought was not about re-defining the flags (I think it is better to keep them intact),
> > > but adding a union for one of rx-only fields (packet_type/rss/timestamp).
> > 
> > Ok. Adding a union field at packet_type field is also fine like below. 
> > 
> > @@ -187,9 +187,10 @@ extern "C" {
> >  /* add new RX flags here, don't forget to update PKT_FIRST_FREE */
> >  
> >  #define PKT_FIRST_FREE (1ULL << 23)
> > -#define PKT_LAST_FREE (1ULL << 40)
> > +#define PKT_LAST_FREE (1ULL << 39)
> >  
> >  /* add new TX flags here, don't forget to update PKT_LAST_FREE  */
> > +#define PKT_TX_MARK_EN		(1ULL << 40)
> >  
> >  /**
> >   * Outer UDP checksum offload flag. This flag is used for enabling
> > @@ -461,6 +462,14 @@ enum {
> >  #endif
> >  };
> >  
> > +/* Tx packet marking flags in rte_mbuf::tx_mark.
> > + * Valid only when PKT_TX_MARK_EN is set in
> > + * rte_mbuf::ol_flags.
> > + */
> > +#define TX_MARK_VLAN_DEI	(1ULL << 0)
> > +#define TX_MARK_IP_DSCP	(1ULL << 1)
> > +#define TX_MARK_IP_ECN		(1ULL << 2)
> > +
> >  /**
> >   * The generic rte_mbuf, containing a packet mbuf.
> >   */
> > @@ -543,6 +552,10 @@ struct rte_mbuf {
> >  			};
> >  			uint32_t inner_l4_type:4; /**< Inner L4 type. */
> >  		};
> > +		struct {
> > +			uint32_t reserved:29;
> > +			uint32_t tx_mark:3;
> > +		};
> >  	};
> > 
> > 
> > 
> > Please correct me if this is not what you mean.
> 
> I'm not a big fan of reusing Rx fields or flags for Tx.
> It's not obvious for an application than adding a tx_mark will overwrite
> the packet_type. I understand that the risk is limited because packet_type
> is Rx and the marks are Tx, but there is still one.

I'm also not a big fan but just wanted to take this approach so that,
it can both conserve space and also help fast path.
Reusing Rx area is however not a new thing as is already followed for 
mbuf->txadapter field.

Apart from documentation issue, Is there any other issue or future 
ramification with using Rx field's for Tx ?
If it is only about documentation, then we can add more documentation to make things clear.

> 
> To summarize the different proposed approaches (please correct me if I'm wrong):
> 
> a- add 3 Tx mbuf flags
>    (-) consumes limited resource
> 
> b- add 3 dynamic flags
>    (-) slower

- Tx burst Vector implementation can't be done for this tx offload as
  offset keeps changing.

> 
> c- add 1 Tx flag and union with Rx field
>    (-) exclusive with Rx field
>    (-) still consumes one flag
> 
> My preference is still b-, for these reasons:
> 
> - There are many different DPDK use cases, and resources in mbuf is tight.
>   Recent contributions (rte_flow and ice driver) already made use of dynamic
>   fields/flags.
- Since RTE_FLOW metadata is 32-bit field, it is a clear candidate for
dynamic flags. 
- ICE PMD's dynamic field is however a vendor specific field and only for
ICE PMD users.

In this case, it is just 1 bit out of 18 free bits available in ol_flags.

> 
> - When I implemented the dynamic fields/flags feature, I did a test which
>   showed that the cost of having a dynamic offset was few cycles (on my test
>   platform, it was~3 cycles for reading a field and ~2 cycles for writing a
>   field).

I think this cost is of the case where the address where the dyn_offset is 
stored is already in cache as it needs to be read first.


> 
> Regards,
> Olivier
> 
> > 
> > > 
> > > > 
> > > > + Techboard
> > > > 
> > > > There is a related thread going on
> > > > https://urldefense.proofpoint.com/v2/url?u=http-3A__mails.dpdk.org_archives_dev_2020-2DMay_168810.html&d=DwIGaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r=FZ_tPCbgFOh18zwRPO9H0yDx8VW38vuapifdDfc8SFQ&m=nyV4Rud03HW6DbWMpyvOCulQNkagmfo0wKtrwQ7zmmg&s=VuktoUb_xoLsHKdB9mV87x67cP9tXk3DqVXptt9nF_s&e= 
> > > > 
> > > > If there is no consensus on email, then I would like to add this item
> > > > to the next TB meeting.
> > > 
> > > Ok, I'll add that to tomorrow meeting agenda.
> > > Konstantin
> > > 
> 
>