Introduce concept of conformance groups

The discussion of what the actual conformance groups should be
is still in progress, so this is just part 1 which only uses
"legacy" for deprecated instructions and "basic" for everything
else. Subsequent patches will add more groups as discussion
continues.

Signed-off-by: Dave Thaler <[e-mail]>
Acked-by: David Vernet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

Documentation/bpf/standardization/instruction-set.rst

···
     A:          10000110
     B: 11111111 10000110
 
+Conformance groups
+------------------
+
+An implementation does not need to support all instructions specified in this
+document (e.g., deprecated instructions).  Instead, a number of conformance
+groups are specified.  An implementation must support the "basic" conformance
+group and may support additional conformance groups, where supporting a
+conformance group means it must support all instructions in that conformance
+group.
+
+The use of named conformance groups enables interoperability between a runtime
+that executes instructions, and tools as such compilers that generate
+instructions for the runtime.  Thus, capability discovery in terms of
+conformance groups might be done manually by users or automatically by tools.
+
+Each conformance group has a short ASCII label (e.g., "basic") that
+corresponds to a set of instructions that are mandatory.  That is, each
+instruction has one or more conformance groups of which it is a member.
+
+The "basic" conformance group includes all instructions defined in this
+specification unless otherwise noted.
+
 Instruction encoding
 ====================
 
···
 
 BPF previously introduced special instructions for access to packet data that were
 carried over from classic BPF. However, these instructions are
-deprecated and should no longer be used.
+deprecated and should no longer be used.  All legacy packet access
+instructions belong to the "legacy" conformance group instead of the "basic"
+conformance group.