Re: [RFC PATCH v4 3/3] ethdev: introduce the cache stashing hints API

[Stephen Hemminger <stephen@networkplumber.org>]

On Sat, 17 May 2025 15:17:35 +0000
Wathsala Vithanage <wathsala.vithanage@arm.com> wrote:

> Extend the ethdev library to enable the stashing of different data
> objects, such as the ones listed below, into CPU caches directly
> from the NIC.
> 
> - Rx/Tx queue descriptors
> - Rx packets
> - Packet headers
> - packet payloads
> - Data of a packet at an offset from the start of the packet
> 
> The APIs are designed in a hardware/vendor agnostic manner such that
> supporting PMDs could use any capabilities available in the underlying
> hardware for fine-grained stashing of data objects into a CPU cache
> 
> The API provides an interface to query the availability of stashing
> capabilities, i.e., platform/NIC support, stashable object types, etc,
> via the rte_eth_dev_stashing_capabilities_get interface.
> 
> The function pair rte_eth_dev_stashing_rx_config_set and
> rte_eth_dev_stashing_tx_config_set sets the stashing hint (the CPU, 
> cache level, and data object types) on the Rx and Tx queues.
> 
> PMDs that support stashing must register their implementations with the
> following eth_dev_ops callbacks, which are invoked by the ethdev
> functions listed above.
> 
> - stashing_capabilities_get
> - stashing_rx_hints_set
> - stashing_tx_hints_set
> 
> Signed-off-by: Wathsala Vithanage <wathsala.vithanage@arm.com>
> Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> Reviewed-by: Dhruv Tripathi <dhruv.tripathi@arm.com>

Adding more control in DPDK is good but I have concerns.

The default must be that the application has all the caching enabled
without calling this API. It would be bad if existing DPDK applications
from network vendors had to be modified.

The DPDK should follow the "it just works" mantra and additional
API's should be for special cases.