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

[Thomas Monjalon <thomas@monjalon.net>]

07/01/2021 17:57, Alexander Kozyrev:
> > 07/01/2021 16:22, Alexander Kozyrev:
> > > > 07/01/2021 16:10, Alexander Kozyrev:
> > > > > > > > Thursday, January 7, 2021 10:18, 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.
> > > >
> > > > Now I'm confused.
> > > > You mean rte_flow_action_copy_data.offset is a bit offset?
> > >
> > > rte_flow_action_copy_data.offset and rte_flow_action_copy_field.width
> > > are measured in bits, right.
> > 
> > So the offset is limited to 16 bits?
> > How can it be useful? Is it an offset starting from the specified field?
> 
> Why 16? It can be up to 2^16=65536 bits. Do you think that is not enough?

Yes 8KB may be too small for huge packets.
I recommend 32 bits.

> And it starts from the specific packet field pointed by the Field ID, correct.

I think it would be more useful as a global offset
starting from the first bit of the packet.


> > > > > > > > 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.
> > > >
> > >
> > > > We can set field and index to 0, and use only offset?
> > > Yes, I'm not inending to put any restrictions against that.
> > > > Then it is a byte offset from the beginning mbuf.data?
> > > Yes, but it is still bit offset, not byte offset.