ISA extension: BPF_CALL | BPF_X

[dthaler]

clang has generated this invalid instruction with -O0 and -O1 for ages.
Allowing its use with something other than the meaning clang gives it might be problematic, though clang's meaning is intuitive.
Should the IANA registry have an entry for it or not?

[dthaler]

Question posted to list at https://mailarchive.ietf.org/arch/msg/bpf/l5tNEgL-Wo7qSEuaGssOl5VChKk/

[dthaler]

Jose Marchesi wrote:

GCC also generates BPF_CALL|BPF_X also named callx, but only if the
experimental -mxbpf option is passed to the compiler.

I recommend this particular encoding to be specifically reserved for a
future `call REG' for when/if a time comes when the BPF verifier
supports some form of indirect calls.

Alexei wrote:

+1.
Same thinking from llvm pov.
CALL|X is what we will use when the kernel supports indirect calls.
I think it means we need to add a 'reserved' category to the spec.

[dthaler]

This also now makes me think we should probably rename the "legacy" conformance group to "packet" for similar reasons. It's the status (Historical) of the group rather than the name that actually makes it legacy.

[dthaler]

Proposed patch posted at https://mailarchive.ietf.org/arch/msg/bpf/Wp4kYteVkk5h1vPxnckQZela8lQ/