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

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Apr 28, 2026 at 06:27:06PM +0000, David Matlack wrote:
> 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?

Yes, it makes more sense to explicitly match the stems through the
flow than to have the implicit $(TEST_GEN_PROGS) == XXX/$(OUTPUT)

> >  	$(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. 

Yes

> 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.

Hmm now that I think about it, I wonder if it used the right flags..

> 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.

Yeah probably

Jason