RE: [RFC 0/4] ethdev: rework config restore

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> >>>> We have been working on optimizing the latency of calls to
> >>>> rte_eth_dev_start(), on ports spawned by mlx5 PMD. Most of the work
> >>>> requires changes in the implementation of
> >>>> .dev_start() PMD callback, but I also wanted to start a discussion
> >>>> regarding configuration restore.
> >>>>
> >>>> rte_eth_dev_start() does a few things on top of calling .dev_start() callback:
> >>>>
> >>>> - Before calling it:
> >>>>     - eth_dev_mac_restore() - if device supports
> >>>> RTE_ETH_DEV_NOLIVE_MAC_ADDR;
> >>>> - After calling it:
> >>>>     - eth_dev_mac_restore() - if device does not support
> >>> RTE_ETH_DEV_NOLIVE_MAC_ADDR;
> >>>>     - restore promiscuous config
> >>>>     - restore all multicast config
> >>>>
> >>>> eth_dev_mac_restore() iterates over all known MAC addresses - stored
> >>>> in rte_eth_dev_data.mac_addrs array - and calls
> >>>> .mac_addr_set() and .mac_addr_add() callbacks to apply these MAC addresses.
> >>>>
> >>>> Promiscuous config restore checks if promiscuous mode is enabled or
> >>>> not, and calls .promiscuous_enable() or .promiscuous_disable() callback.
> >>>>
> >>>> All multicast config restore checks if all multicast mode is enabled
> >>>> or not, and calls .allmulticast_enable() or .allmulticast_disable() callback.
> >>>>
> >>>> Callbacks are called directly in all of these cases, to bypass the
> >>>> checks for applying the same configuration, which exist in relevant APIs.
> >>>> Checks are bypassed to force drivers to reapply the configuration.
> >>>>
> >>>> Let's consider what happens in the following sequence of API calls.
> >>>>
> >>>> 1. rte_eth_dev_configure()
> >>>> 2. rte_eth_tx_queue_setup()
> >>>> 3. rte_eth_rx_queue_setup()
> >>>> 4. rte_eth_promiscuous_enable()
> >>>>     - Call dev->dev_ops->promiscuous_enable()
> >>>>     - Stores promiscuous state in dev->data->promiscuous 5.
> >>>> rte_eth_allmulticast_enable()
> >>>>     - Call dev->dev_ops->allmulticast_enable()
> >>>>     - Stores allmulticast state in dev->data->allmulticast 6.
> >>>> rte_eth_dev_start()
> >>>>     - Call dev->dev_ops->dev_start()
> >>>>     - Call dev->dev_ops->mac_addr_set() - apply default MAC address
> >>>>     - Call dev->dev_ops->promiscuous_enable()
> >>>>     - Call dev->dev_ops->allmulticast_enable()
> >>>>
> >>>> Even though all configuration is available in dev->data after step 5,
> >>>> library forces reapplying this configuration in step 6.
> >>>>
> >>>> In mlx5 PMD case all relevant callbacks require communication with the
> >>>> kernel driver, to configure the device (mlx5 PMD must create/destroy
> >>>> new kernel flow rules and/or change netdev config).
> >>>>
> >>>> mlx5 PMD handles applying all configuration in .dev_start(), so the
> >>>> following forced callbacks force additional communication with the kernel. The
> >>> same configuration is applied multiple times.
> >>>>
> >>>> As an optimization, mlx5 PMD could check if a given configuration was
> >>>> applied, but this would duplicate the functionality of the library
> >>>> (for example rte_eth_promiscuous_enable() does not call the driver if
> >>>> dev->data->promiscuous is set).
> >>>>
> >>>> Question: Since all of the configuration is available before
> >>>> .dev_start() callback is called, why ethdev library does not expect .dev_start() to
> >>> take this configuration into account?
> >>>> In other words, why library has to reapply the configuration?
> >>>>
> >>>> I could not find any particular reason why configuration restore
> >>>> exists as part of the process (it was in the initial DPDK commit).
> >>>>
> >>>
> >>> My assumption is .dev_stop() cause these values reset in some devices, so
> >>> .dev_start() restores them back.
> >>> @Bruce or @Konstantin may remember the history.
> >
> > Yep, as I remember, at least some Intel PMDs calling hw_reset() ad dec_stop() and
> > even dev_start() to make sure that HW is in a clean (known) state.
> >
> >>>
> >>> But I agree this is device specific behavior, and can be managed by what device
> >>> requires.
> >
> > Probably yes.
> >
> >>>
> >>>> The patches included in this RFC, propose a mechanism which would help
> >>>> with managing which drivers rely on forceful configuration restore.
> >>>> Drivers could advertise if forceful configuration restore is needed
> >>>> through `RTE_ETH_DEV_*_FORCE_RESTORE` device flag. If this flag is
> >>>> set, then the driver in question requires ethdev to forcefully restore
> >>> configuration.
> >>>>
> >>>
> >>> OK to use flag for it, but not sure about using 'dev_info->dev_flags'
> >>> (RTE_ETH_DEV_*) for this, as this flag is shared with user and this is all dpdk
> >>> internal.
> >>>
> >>> What about to have a dedicated flag for it? We can have a dedicated set of flag
> >>> values for restore.
> >>
> >> Agreed. What do you think about the following?
> >
> > Instead of exposing that, can we probably make it transparent to the user
> > and probably ethdev layer too?
> >
> 
> +1 to make it transparent to user, but not sure if we can make it
> transparent to ethdev layer.

Just to be clear:
Let say, using example from above: 

 rte_eth_dev_start()
     - Call dev->dev_ops->dev_start()
     - Call dev->dev_ops->mac_addr_set() - apply default MAC address
     - Call dev->dev_ops->promiscuous_enable()
     - Call dev->dev_ops->allmulticast_enable()

We probably can introduce ethdev internal function (still visible to PMDs)
that would do last 3 steps:
ethdev_replay_user_conf(...)
     - Call dev->dev_ops->mac_addr_set() - apply default MAC address
     - Call dev->dev_ops->promiscuous_enable()
     - Call dev->dev_ops->allmulticast_enable() 

And let PMD itself to decide does it needs to call it at dev_start() or not.
So it will become:
rte_eth_dev_start()
     - Call dev->dev_ops->dev_start()
	-Call ethdev_replay_user_conf(.)
     		- Call dev->dev_ops->mac_addr_set() - apply default MAC address
     		- Call dev->dev_ops->promiscuous_enable()
     		-Call dev->dev_ops->allmulticast_enable()

For PMDs that do need to restore user provided config
And 
rte_eth_dev_start()
     - Call dev->dev_ops->dev_start()

For those who do not.

> Suggested 'internal_flag' in "struct rte_eth_dev_data" can be confusing
> and open to interpretation what to use it for and by time become source
> of defect.

Yes, same thoughts.

> Instead what do you think to have a separate, dedicated data struct for it?

Hmm... not sure I understood you here...

> 
> > Might be we can move this restoration code into the new ethdev helper function,(ethdevd_user_config_restore()  or so)
> > that PMD can invoke during its dev_start() if needed?
> >
> >>
> >> #define RTE_ETH_DEV_INTERNAL_PROMISC_FORCE_RESTORE RTE_BIT32(0)
> >> #define RTE_ETH_DEV_INTERNAL_ALLMULTI_FORCE_RESTORE RTE_BIT32(1)
> >> #define RTE_ETH_DEV_INTERNAL_MAC_ADDR_FORCE_RESTORE RTE_BIT32(2)
> >>
> >> struct rte_eth_dev_data {
> >> 	/* snip */
> >>
> >> 	uint32_t dev_flags;
> >>
> >> 	/**
> >> 	 * Internal device capabilities, used only by ethdev library.
> >> 	 * Certain functionalities provided by the library might enabled/disabled,
> >> 	 * based on driver exposing certain capabilities.
> >> 	 */
> >> 	uint32_t internal_flags;
> >>
> >> 	/* snip */
> >> };
> >>
> >>> Also perhaps we have go into details what needs to be restored after 'stop' and
> >>> what needs to be restored after 'reset' and use similar mechanism etc...
> >>
> >> I think we should look into that.
> >> Any 'codification' of semantics between drivers and ethdev library is good in my opinion.
> >>
> >> At least right now, ethdev does not change any configuration in 'stop' and 'reset' from what I see.
> >> But that's on library side only.
> >>
> >>>> This way, if we would conclude that it makes sense for .dev_start() to
> >>>> handle all starting configuration aspects, we could track which drivers still rely
> >>> on configuration restore.
> >>>>
> >>>> Dariusz Sosnowski (4):
> >>>>   ethdev: rework config restore
> >>>>   ethdev: omit promiscuous config restore if not required
> >>>>   ethdev: omit all multicast config restore if not required
> >>>>   ethdev: omit MAC address restore if not required
> >>>>
> >>>>  lib/ethdev/rte_ethdev.c | 39 ++++++++++++++++++++++++++++++++++-----
> >>>>  lib/ethdev/rte_ethdev.h | 18 ++++++++++++++++++
> >>>>  2 files changed, 52 insertions(+), 5 deletions(-)
> >>>>
> >>>> --
> >>>> 2.39.5
> >>>>
> >