Use 0 as default PSA_PORT_RECIRCULATE value (#3360)

p4lang · May 25, 2022 · 2bffff8 · 2bffff8
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diff --git a/backends/ebpf/psa/README.md b/backends/ebpf/psa/README.md
@@ -435,21 +435,34 @@ $ python3 backends/ebpf/build_libbpf
 You can compile a P4-16 PSA program for eBPF in a single step using:
 
 ```bash
-make -f backends/ebpf/runtime/kernel.mk BPFOBJ=out.o ARGS="-DPSA_PORT_RECIRCULATE=<RECIRCULATE_PORT_IDX>" P4FILE=<P4-PROGRAM>.p4 P4C=p4c-ebpf psa
+make -f backends/ebpf/runtime/kernel.mk BPFOBJ=out.o P4FILE=<P4-PROGRAM>.p4 P4C=p4c-ebpf psa
 ```
 
 You can also perform compilation step by step:
 
 ```
 $ p4c-ebpf --arch psa --target kernel -o out.c <program>.p4
-$ clang -Ibackends/ebpf/runtime -Ibackends/ebpf/runtime/usr/include -O2 -g -c -emit-llvm -DBTF -DPSA_PORT_RECIRCULATE=<RECIRCULATE_PORT_IDX> -o out.bc out.c
+$ clang -Ibackends/ebpf/runtime -Ibackends/ebpf/runtime/usr/include -O2 -g -c -emit-llvm -o out.bc out.c
 $ llc -march=bpf -mcpu=generic -filetype=obj -o out.o out.bc
 ```
 
 Note that you can use `-mcpu` flag to define the eBPF instruction set. Visit [this blog post](https://pchaigno.github.io/bpf/2021/10/20/ebpf-instruction-sets.html) to learn more about eBPF instruction sets.
 
 The above steps generate `out.o` BPF object file that can be loaded to the kernel. 
 
+#### Optional flags
+
+Supposing we want to use a packet recirculation we have to specify the `PSA_PORT_RECIRCULATE` port.
+We can use `-DPSA_PORT_RECIRCULATE=<RECIRCULATE_PORT_IDX>` Clang flag via `kernel.mk`
+```bash
+make -f backends/ebpf/runtime/kernel.mk BPFOBJ=out.o ARGS="-DPSA_PORT_RECIRCULATE=<RECIRCULATE_PORT_IDX>" P4FILE=<P4-PROGRAM>.p4 P4C=p4c-ebpf psa
+```
+or directly:
+`clang ... -DPSA_PORT_RECIRCULATE=<RECIRCULATE_PORT_IDX> ...`,  
+where `RECIRCULATE_PORT_IDX` is a number of a `psa_recirc` interface (this number can be obtained from `ip -n switch link`).
+
+By default `PSA_PORT_RECIRCULATE` is set to 0.
+
 ### psabpf API and psabpf-ctl
 
 We provide the `psabpf` C API and the `psabpf-ctl` CLI tool that can be used to manage eBPF programs generated by P4-eBPF compiler.

diff --git a/backends/ebpf/psa/ebpfPsaGen.cpp b/backends/ebpf/psa/ebpfPsaGen.cpp
@@ -79,9 +79,7 @@ void PSAEbpfGenerator::emitPreamble(CodeBuilder *builder) const {
     builder->newline();
 
     builder->appendLine("#ifndef PSA_PORT_RECIRCULATE\n"
-        "#error \"PSA_PORT_RECIRCULATE not specified, "
-        "please use -DPSA_PORT_RECIRCULATE=n option to specify index of recirculation "
-        "interface (see the result of command 'ip link')\"\n"
+        "#define PSA_PORT_RECIRCULATE 0\n"
         "#endif");
     builder->appendLine("#define P4C_PSA_PORT_RECIRCULATE 0xfffffffa");
     builder->newline();