Re: [dpdk-dev] [PATCH v2] ethdev: document RSS default key and types

[Ophir Munk <ophirmu@mellanox.com>]

> -----Original Message-----
> From: Adrien Mazarguil [mailto:adrien.mazarguil@6wind.com]
> Sent: Wednesday, November 07, 2018 4:06 PM
> To: Ophir Munk <ophirmu@mellanox.com>
> Cc: Ferruh Yigit <ferruh.yigit@intel.com>; Andrew Rybchenko
> <arybchenko@solarflare.com>; dev@dpdk.org; Thomas Monjalon
> <thomas@monjalon.net>; Asaf Penso <asafp@mellanox.com>; Shahaf
> Shuler <shahafs@mellanox.com>; Olga Shern <olgas@mellanox.com>
> Subject: Re: [dpdk-dev] [PATCH v2] ethdev: document RSS default key and
> types
> 
> On Wed, Nov 07, 2018 at 12:39:24PM +0000, Ophir Munk wrote:
> > > -----Original Message-----
> > > From: Adrien Mazarguil [mailto:adrien.mazarguil@6wind.com]
> > > Sent: Wednesday, November 07, 2018 11:31 AM
> > > To: Ophir Munk <ophirmu@mellanox.com>
> > > Cc: Ferruh Yigit <ferruh.yigit@intel.com>; Andrew Rybchenko
> > > <arybchenko@solarflare.com>; dev@dpdk.org; Thomas Monjalon
> > > <thomas@monjalon.net>; Asaf Penso <asafp@mellanox.com>; Shahaf
> > > Shuler <shahafs@mellanox.com>; Olga Shern <olgas@mellanox.com>
> > > Subject: Re: [dpdk-dev] [PATCH v2] ethdev: document RSS default key
> > > and types
> > >
> > > On Wed, Nov 07, 2018 at 09:23:42AM +0000, Ophir Munk wrote:
> > > > struct rte_flow_action_rss include fields 'key' and 'types'.
> > > > Field 'key' is a pointer to bytes array (uint8_t *) which contains
> > > > the specific RSS hash key.
> > > > If an application is only interested in default RSS operation it
> > > > should not care about the specific hash key. The application can
> > > > set the hash key to NULL such that any PMD uses its default RSS key.
> > > >
> > > > Field 'types' is a uint64_t bits flag used to specify a specific
> > > > RSS hash type such as ETH_RSS_IP (see ETH_RSS_*).
> > > > If an application does not care about the specific RSS type it can
> > > > set this field to 0 such that any PMD uses its default type.
> > > >
> > > > Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
> > > > ---
> > > >  lib/librte_ethdev/rte_flow.h | 9 +++++++--
> > > >  1 file changed, 7 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/lib/librte_ethdev/rte_flow.h
> > > > b/lib/librte_ethdev/rte_flow.h index c0fe879..ca9e135 100644
> > > > --- a/lib/librte_ethdev/rte_flow.h
> > > > +++ b/lib/librte_ethdev/rte_flow.h
> > > > @@ -1782,10 +1782,15 @@ struct rte_flow_action_rss {
> > > >  	 * through.
> > > >  	 */
> > > >  	uint32_t level;
> > > > -	uint64_t types; /**< Specific RSS hash types (see ETH_RSS_*). */
> > > > +	/**
> > > > +	 * Specific RSS hash types (see ETH_RSS_*),
> > > > +	 * or 0 for PMD specific default.
> > > > +	 */
> > > > +	uint64_t types;
> > > >  	uint32_t key_len; /**< Hash key length in bytes. */
> > > >  	uint32_t queue_num; /**< Number of entries in @p queue. */
> > > > -	const uint8_t *key; /**< Hash key. */
> > > > +	/** Hash key, or NULL for PMD specific default key. */
> > > > +	const uint8_t *key;
> > >
> > > I'd suggest to document that on key_len instead. If key_len is
> > > nonzero, key cannot be NULL anyway.
> >
> > The decision if a key/len combination is valid is done in the PMD action
> validation API.
> > For example, in MLX5 key==NULL and key_len==40 is accepted.
> > The combination key==NULL and key_len==0 should always succeeds,
> however the "must" parameter for RSS default is key==NULL and not
> key_len==0.
> 
> I understand this is how the mlx5 PMD implemented it, but my point is that it
> makes more sense API-wise to define key_len == 0 as the trigger for a default
> RSS hash key than key == NULL.
> 
> My suggestion is to follow the same trend as memcpy(), mmap(), snprintf()
> and other well-known functions that take a size when dealing with
> NULL/undefined pointers. Only size matters! :)
> 

Please let's stay backward compatible and consistent with previous dpdk releases where
key==NULL is used in struct rte_eth_rss_conf (see code snippet in [1]).
We should avoid confusing applications with setting key==NULL with legacy RSS and key_len==0 with rte_flows.

With regard to trends APIs - I was thinking about free() where a NULL pointer is acceptable :)

[1]
File lib/librte_ethdev/rte_ethdev.h:

/**
 * A structure used to configure the Receive Side Scaling (RSS) feature
 * of an Ethernet port.
 * If not NULL, the *rss_key* pointer of the *rss_conf* structure points
 * to an array holding the RSS key to use for hashing specific header
 * fields of received packets. The length of this array should be indicated
 * by *rss_key_len* below. Otherwise, a default random hash key is used by
 * the device driver.
 *
 * The *rss_key_len* field of the *rss_conf* structure indicates the length
 * in bytes of the array pointed by *rss_key*. To be compatible, this length
 * will be checked in i40e only. Others assume 40 bytes to be used as before.
 *
 * ..........
 */
struct rte_eth_rss_conf {
	uint8_t *rss_key;    /**< If not NULL, 40-byte hash key. */
	uint8_t rss_key_len; /**< hash key length in bytes. */
	uint64_t rss_hf;     /**< Hash functions to apply - see below. */
};

> --
> Adrien Mazarguil
> 6WIND