Re: [dpdk-dev] [PATCH 0/9] net/cxgbe: updates for rte_flow support

[Thomas Monjalon <thomas@monjalon.net>]

18/03/2020 14:06, Rahul Lakkireddy:
> Hi Thomas,
> 
> On Wednesday, March 03/18/20, 2020 at 13:09:47 +0100, Thomas Monjalon wrote:
> > 11/03/2020 10:05, Rahul Lakkireddy:
> > > From: Karra Satwik <kaara.satwik@chelsio.com>
> > > 
> > > This series of patches contain rte_flow support for matching
> > > Q-in-Q VLAN, IP TOS, PF, and VF fields. Also, adds Destination
> > > MAC rewrite and Source MAC rewrite actions.
> > > 
> > > Apart from the 4-tuple (IP src/dst addresses and TCP/UDP src/dst
> > > port addresses), there are only 40-bits available to match other
> > > fields in packet headers. Currently, the combination of packet
> > > header fields to match are configured via filterMode for LETCAM
> > > filters and filterMask for HASH filters in firmware config files
> > > (t5/t6-config.txt). Adapter needs to be reflashed with new firmware
> > > config file everytime the combinations need to be changed. To avoid
> > > this, a new firmware API is available to dynamically change the
> > > combination before completing full adapter initialization. So, 2
> > > new devargs filtermode and filtermask are added to dynamically
> > > select the combinations during runtime.
> > 
> > Please, could you explain why you are using devargs for flow matching,
> > instead of using the common and generic rte_flow API?
> 
> The devargs are being used to configure the TCAM in hardware on
> what header fields need to be matched in packets by the TCAM. The
> actual filter rules are still being inserted using rte_flow API.
> 
> Apart from the 4-tuple (src/dst IP, src/dst port addresses), there
> are only 40-bits available for each filter rule to match other
> header fields. Hardware supports matching ethertype (16-bit),
> DST MAC (9-bit MPS index), Inner VLAN (16-bit), Outer VLAN (16-bit),
> IP Protocol (8-bit), IP TOS (8-bit), Ingress Physical Port (3-bit),
> and PFVF (17-bit) for now. It's not possible to write a filter rule
> which wants to match all the above fields, which is far beyond 40-bits
> available. So, the devargs are being used to control which of the
> above fields that user wants to configure the TCAM to match. Note
> that once the TCAM is configured, "all" the rules in the TCAM can
> only match the selected fields in the combination. They can't match
> any other header fields.

In case a rule is not possible, are you rejecting it at the validation stage?

> For example, let's say user wants to match ethertype (16-bit),
> DST MAC (9-bit MPS index), and IP protocol (8-bit) in all filter
> rules. Then, they would configure the TCAM with the {ethertype,
> DST MAC, and IP protocol} combination. All rules that the user
> wants to insert into TCAM can only have the above header fields,
> alongside the default 4-tuple (src/dst IP, src/dst port addresses).
> 
> There are 2 regions in hardware. One for matching wild-card filter
> rules and the other for matching exact-match rules. The "filtermode"
> devarg controls the 40-bit combination for wild-card filter rules and
> the "filtermask" devarg controls the combination for exact-match rules.

I see an issue with this approach. I will explain below.

An application is written to use some flow rules.
The application requirements are expressed by the app developper
through the API (rte_flow).
In your case, the user must be aware of what the application expects
and fill the right devargs, according to what the dev wrote.
Why bothering the user with this constraint?

I understand the hardware must be prepared in advance.
I think this configuration must be done through API.
One workaround is to manage this HW limitation in a PMD-specific API.
A good solution would be to express this requirement with rte_flow.

One idea: can we use rte_flow_validate() to fill the requirements?
The PMD requirements are empty at the beginning and they are filled
with the first calls to rte_flow_validate().

Maybe we also need to express the capabilities/limitations.
Example: is there a maximum number of rules?
maximum number of protocols to match?
maximum number of bits to match?

I suppose it is not easy to implement. Comments?