Re: [dpdk-dev] [RFC] ethdev: introduce copy_field rte flow action

[Alexander Kozyrev <akozyrev@nvidia.com>]

> > > Thursday, January 7, 2021 10:07, Thomas Monjalon <thomas@monjalon.net>
> > > > RTE Flows API lacks the ability to save an arbitrary header field in
> > > > order to use it later for advanced packet manipulations. Examples
> > > > include the usage of VxLAN ID after the packet is decapsulated or
> > > > storing this ID inside the packet payload itself or swapping an
> > > > arbitrary inner and outer packet fields.
> > > >
> > > > The idea is to allow a copy of a specified number of bits form any
> > > > packet header field into another header field:
> > > > RTE_FLOW_ACTION_TYPE_COPY_FIELD with the structure defined below.
> > > >
> > > > struct rte_flow_action_copy_field {
> > > > 	struct rte_flow_action_copy_data dest;
> > > > 	struct rte_flow_action_copy_data src;
> > > > 	uint16_t width;
> > > > };
> > > >
> > > > Arbitrary header field (as well as mark, metadata or tag values) can be
> > > > used as both source and destination fields. This way we can save an
> > > > arbitrary header field by copying its value to a tag/mark/metadata or
> > > > copy it into another header field directly. tag/mark/metadata can also
> > > > be used as a value to be stored in an arbitrary packet header field.
> > > >
> > > > struct rte_flow_action_copy_data {
> > > > 	enum rte_flow_field_id field;
> > > > 	uint16_t index;
> > > > 	uint16_t offset;
> > > > };
> > > >
> > > > The rte_flow_field_id specifies the particular packet field (or
> > > > tag/mark/metadata) to be used as a copy source or destination.
> > > > The index gives access to inner packet headers or elements in the tags
> > > > array. The offset allows to copy a packet field value into the payload.
> > >
> > > So index is in reality the layer? How is it numbered exactly?
> >
> > It is a layer for packet fields, inner headers get higher number index.
> > But is it also an index in the TAG array, so the name comes from it.
> 
> Sorry it is not obvious.
> Please describe the exact numbering in tunnel and VLAN cases.
> 
> > > What is the field id if an offset is given?
> >
> > Field ID stays the same, you can specify a small offset to copy just a few bits
> > from the entire packet field or a big offset to move to completely different
> area.
> 
> I don't understand what is an offset then.
> Isn't it the byte or bit where the copy start?
> Do you handle sizes smaller than a byte?
It is the bit offset, you can copy 20 bits out of 32 bits of IPv4 address for example.
 
> > > Can we say that a field id can always be replaced by an offset?
> >
> > Not really. You can use offset to jump around packet fields for sure, but it is
> going to be
> > hard and cumbersome to calculate all the offsets for that. Field ID is much
> more convenient.
> 
> I think it depends for who.
> For some use cases, it may be easier to pass an offset.
> For some drivers, it may be more efficient to directly manage offsets.
It is possible with this RFC, driver can choose what to use: id and/or offset.