Subject: RE: [PATCH net-next v8 10/15] p4tc: add action template create, update, delete, get, flush and dump

From: John Fastabend <[email-masked]>
Date: Thu, 16 Nov 2023 22:51:45 -0800

Jamal Hadi Salim wrote:
> This commit allows users to create, update, delete, get, flush and dump
> dynamic action kinds based on P4 action definition.
> 
> At the moment dynamic actions are tied to P4 programs only and cannot be
> used outside of a P4 program definition.
> 
> Visualize the following action in a P4 program:
> 
> action ipv4_forward(@tc_type("macaddr) bit<48> dstAddr, @tc_type("dev") bit<8> port)
> {
>      // Action code (generated by the compiler)

So this is BPF or what?

> }
> 
> The above is an action called ipv4_forward which receives as parameters
> a bit<48> dstAddr (a mac address) and a bit<8> port (something close to
> ifindex).
> 
> which is invoked on a P4 table match as such:
> 
> table mytable {
>         key = {
>             hdr.ipv4.dstAddr @tc_type("ipv4"): lpm;
>         }
> 
>         actions = {
>             ipv4_forward;
>             drop;
>             NoAction;
>         }
> 
>         size = 1024;
> }
> 
> We don't have an equivalent built in "ipv4_forward" action in TC. So we
> create this action dynamically.
> 
> The mechanics of dynamic actions follow the CRUD semantics.
> 
> ___DYNAMIC ACTION KIND CREATION___
> 
> In this stage we issue the creation command for the dynamic action which
> specifies the action name, its ID, parameters and the parameter types.
> So for the ipv4_forward action, the creation would look something like
> this:
> 
> tc p4template create action/aP4proggie/ipv4_forward \
>   param dstAddr type macaddr id 1 param port type dev id 2
> 
> Note1: Although the P4 program defined dstAddr as type bit48 we use our
> type called macaddr (likewise for port) - see commit on p4 types for
> details.
> 
> Note2: All the template commands (tc p4template) are generated by the
> p4c compiler.
> 
> Note that in the template creation op we usually just specify the action
> name, the parameters and their respective types. Also see that we specify
> a pipeline name during the template creation command. As an example, the
> above command creates an action template that is bounded to
> pipeline or program named aP4proggie.
> 
> Note, In P4, actions are assumed to pre-exist and have an upper bound
> number of instances. Typically if you have 1M table entries you want to allocate
> enough action instances to cover the 1M entries. However, this is a big waste
> waste of memory if the action instances are not in use. So for our case, we allow
> the user to specify a minimal amount of actions instances in the template and then
> if more dynamic action instances are needed then they will be added on
> demand as in the current approach with tc filter-action relationship.
> For example, if one were to create the action ipv4_forward preallocating
> 128 instances, one would issue the following command:
> 
> tc p4template create action/aP4proggie/ipv4_forward num_prealloc 128 \
>   param dstAddr type macaddr id 1 param port type dev id 2
> 
> By default, 16 action instances will be preallocated.
> If the user wishes to have more actions instances, they will have to be
> created individually by the control plane using the tc actions command.
> For example:
> 
> tc actions add action aP4proggie/ipv4_forward \
> param dstAddr AA:BB:CC:DD:EE:DD param port eth1
> 
> Only then they can issue a table entry creation command using this newly
> created action instance.
> 
> Note, this does not disqualify a user from binding to an existing action
> instances. For example:
> 
> tc p4ctrl create aP4proggie/table/mycontrol/mytable \
>    srcAddr 10.10.10.0/24 action ipv4_forward index 1
> 
> ___ACTION KIND ACTIVATION___
> 
> Once we provided all the necessary information for the new dynamic action,
> we can go to the final stage, which is action activation. In this stage,
> we activate the dynamic action and make it available for instantiation.
> To activate the action template, we issue the following command:
> 
> tc p4template update action aP4proggie/ipv4_forward state active
> 
> After the above the command, the action is ready to be instantiated.
> 
> ___RUNTIME___
> 
> This next section deals with the runtime part of action templates, which
> handle action template instantiation and binding.
> 
> To instantiate a new action that was created from a template, we use the
> following command:
> 
> tc actions add action aP4proggie/ipv4_forward \
> param dstAddr AA:BB:CC:DD:EE:FF param port eth0 index 1
> 
> Observe these are the same semantics as what tc today already provides
> with a caveat that we have a keyword "param" to precede the appropriate
> parameters - as such specifying the index is optional (kernel provides
> one when unspecified).
> 
> As previously stated, we refer to the action by it's "full name"
> (pipeline_name/action_name). Here we are creating an instance of the
> ipv4_forward action specifying as parameter values AA:BB:CC:DD:EE:FF for
> dstAddr and eth0 for port. We can create as many instances for action
> templates as we wish.
> 
> To bind the above instantiated action to a table entry, you can do use the
> same classical approach used to bind ordinary actions to filters, for
> example:
> 
> tc p4ctrl create aP4proggie/table/mycontrol/mytable \
>    srcAddr 10.10.10.0/24 action ipv4_forward index 1
> 
> The above command will bind our newly instantiated action to a table
> entry which is executed if there's a match.
> 
> Of course one could have created the table entry as:
> 
> tc p4ctrl create aP4proggie/table/mycontrol/mytable \
> srcAddr 10.10.10.0/24 \
> action ipv4_forward param dstAddr AA:BB:CC:DD:EE:FF param port eth0
> 
> Actions from other P4 control blocks (in the same pipeline) might be
> referenced as the action index is global within a pipeline.
> 

Where did what the action actually does get defined? It looks like
a label at this point, but without code?