On Mon, Dec 12, 2022, Nick Desaulniers wrote: > > diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile > > index 2487db21b177..9cff99a1cb2e 100644 > > --- a/tools/testing/selftests/kvm/Makefile > > +++ b/tools/testing/selftests/kvm/Makefile > > @@ -196,6 +196,7 @@ else > > LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/$(ARCH)/include > > endif > > CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \ > > + -Wno-gnu-variable-sized-type-not-at-end \ > > This is a clang-specific warning. This will need to be wrapped in a > cc-option check. Not that I'm against guarding this code, but I don't think cc-option() will do anything in this case. AFAICT, gcc stopped treating unknown "-Wno" flags as unconditional errors starting with gcc-4.4, and the kernel's min supported version is 5.1. gcc-4.4 through gcc-9.5 all print a mild warning if there's a different error, but otherwise silently ignore the uknown "-Wno". cc1: warning: unrecognized command line option '-Wno-gnu-variable-sized-type-not-at-end' gcc-10.1 is even friendlier and notes that the unknown flag may have been related to the error. cc1: note: unrecognized command-line option '-Wno-gnu-variable-sized-type-not-at-end' may have been intended to silence earlier diagnostics Because cc-option() doesn't have errors in its probing code, it will return "true" on gcc for literally any "-Wno-*" input that gcc deems syntacially valid, e.g. gcc barfs on depends on $(cc-option,-Wno-) depends on $(cc-option,-Wno) but happily succeeds with depends on $(cc-option,-Wno-lol-gcc) Various man pages suggest -Wunknown-warnings is a thing, but no gcc version supported by godbolt recognizes it. So unless I'm missing something, trying to detect lack of support will be non-trivial, and the worst case scenario is that users of older gcc version will see a potentially confusing warning when the build fails.
