Re: [PATCH] vfio: selftests: Fix out-of-tree build with make O=

[David Matlack <dmatlack@google.com>]

On 2026-04-27 08:07 PM, Jason Gunthorpe wrote:
> The test programs are compiled via a static pattern rule that requires
> intermediate .o files:
> 
>   $(TEST_GEN_PROGS): %: %.o $(LIBVFIO_O)
> 
> After lib.mk prefixes TEST_GEN_PROGS with $(OUTPUT), this creates
> dependencies on .o files in the output directory (e.g.
> $(OUTPUT)/vfio_dma_mapping_test.o). However, there is no rule to compile
> these .o files from the source directory .c files when OUTPUT differs
> from the source directory.
> 
> Add an explicit chain of pattern rules:
>   $(OUTPUT)/% -> $(OUTPUT)/%.o -> %.c
> 
> Following the same pattern already used in libvfio.mk for the library
> objects.
> 
> Fixes: 19faf6fd969c ("vfio: selftests: Add a helper library for VFIO selftests")
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  tools/testing/selftests/vfio/Makefile | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/vfio/Makefile b/tools/testing/selftests/vfio/Makefile
> index 40165d087a0bc4..81c3be8298fdc2 100644
> --- a/tools/testing/selftests/vfio/Makefile
> +++ b/tools/testing/selftests/vfio/Makefile
> @@ -27,10 +27,13 @@ CFLAGS += $(EXTRA_CFLAGS)
>  
>  LDFLAGS += -pthread
>  
> -$(TEST_GEN_PROGS): %: %.o $(LIBVFIO_O)
> +$(TEST_GEN_PROGS): $(OUTPUT)/%: %(OUTPUT)/%.o $(LIBVFIO_O)

Changing this line (with the fix you replied with) is not actually
needed right? I tried the following without it and was able to build:

  $ make OUTPUT=/tmp -C tools/testing/selftests/vfio

$(TEST_GEN_PROGS) has already been prefixed with $(OUTPUT), so %
in the original pattern already included $(OUTPUT).

Is this for readability?

>  	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) $< $(LIBVFIO_O) $(LDLIBS) -o $@
>  
>  TEST_GEN_PROGS_O = $(patsubst %, %.o, $(TEST_GEN_PROGS))
> +$(TEST_GEN_PROGS_O): $(OUTPUT)/%.o: %.c
> +	$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c $< -o $@

Looking at this commit made me confused how this was even working before
and I learned that make has built-in rules to build .o files that
assumes the .c is in the same directory. I see now that we were reying
on those built-in rules to build $(TEST_GEN_PROGS_O). It makes sense to
have an explicit rule to handle the directory mismatch between .c and
.o.

This also got me thinking it might be a nice cleanup to use the
predefined macros $(COMPILE.c) and $(LINK.c) in these rules instead of
$(CC) $(CFLAGS) ... etc. No need to attach that to this fix though.

> +
>  TEST_DEP_FILES = $(patsubst %.o, %.d, $(TEST_GEN_PROGS_O) $(LIBVFIO_O))
>  -include $(TEST_DEP_FILES)
>  
> 
> base-commit: 681b35420592a2e072d9759254185e72d2e914c8
> -- 
> 2.43.0
>