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doc: update flow API guide for rule removal on stop

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There is a discrepancy between ethdev API and flow rules guide
regarding flow rules maintenance after port stop.
librte_ethdev.h declares that flow rules will not be stored in PMD
after port stop:
>>>>> Quote start
 Please note that some configuration is not stored between calls to
 rte_eth_dev_stop()/rte_eth_dev_start(). The following configuration
 will be retained:

 - MTU
 - flow control settings
 - receive mode configuration (promiscuous mode, all-multicast mode,
   hardware checksum mode, RSS/VMDQ settings etc.)
 - VLAN filtering configuration
 - default MAC address
 - MAC addresses supplied to MAC address array
 - flow director filtering mode (but not filtering rules)
 - NIC queue statistics mappings
<<<< Quote end

PMD cannot always correctly restore flow rules after port stop / port
start because application may alter port configuration after port stop
without PMD knowledge about undergoing changes.  Consider the
following scenario:
application configures 2 queues 0 and 1 and creates a flow rule with
'queue index 1' action. After that application stops the port and
removes queue 1.
Although PMD can implement flow rule shadow copy to be used for
restore after port start, attempt to restore flow rule from shadow
will fail in example above and PMD could not notify application about
that failure.  As the result, flow rules map in HW will differ from
what application expects.  In addition, flow rules shadow copy used
for port start restore consumes considerable amount of system memory,
especially in systems with millions of flow rules.

Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Ori Kam <orika@nvidia.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
This commit is contained in:
Gregory Etelson 2020-11-18 18:15:20 +02:00 committed by Thomas Monjalon
parent c57f6e5c60
commit 4467fed6f9
1 changed files with 6 additions and 4 deletions
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10
doc/guides/prog_guide/rte_flow.rst
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@ -3229,10 +3229,12 @@ Caveats
temporarily replacing the burst function pointers), an appropriate error
code must be returned (``EBUSY``).
- PMDs, not applications, are responsible for maintaining flow rules
configuration when stopping and restarting a port or performing other
actions which may affect them. They can only be destroyed explicitly by
applications.
- Applications, not PMDs, are responsible for maintaining flow rules
configuration when closing, stopping or restarting a port or performing other
actions which may affect them.
Applications must assume that after port close, stop or restart all flows
related to that port are not valid, hardware rules are destroyed and relevant
PMD resources are released.
For devices exposing multiple ports sharing global settings affected by flow
rules: