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

[Olivier Matz <olivier.matz@6wind.com>]

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.

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

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.

- 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).

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
> >