Re: [PATCH v2 1/2] KVM: selftests: Fix arm64 sysreg header dependencies

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 22, 2026, Fuad Tabba wrote:
> The rule generating the arm64 sysreg header targets the generated
> directory, and its only prerequisite is tools/arch/arm64/tools/*,
> which expands to the inner Makefile. An existing directory looks up
> to date, so an incremental build never regenerates sysreg-defs.h for
> a change to the sysreg table. A selftest referencing a register added
> to the table since then fails with undeclared SYS_* identifiers. Clean
> builds are unaffected.
> 
> Always recurse into tools/arch/arm64/tools, whose Makefile already
> tracks the generator's inputs, and target the header rather than its
> directory. The recursion leaves the header untouched when nothing
> changed, so the dependents rebuild only when the header does.
> 
> Fixes: 70c7b704ca725 ("KVM: selftests: Avoid using forced target for generating arm64 headers")
> Link: https://lore.kernel.org/r/arGRhHgP_izSwW4T@kernel.org
> Assisted-by: LLM
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
> ---
>  tools/testing/selftests/kvm/Makefile.kvm | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
> index 96bab7002d39e..81dca3dbc16b4 100644
> --- a/tools/testing/selftests/kvm/Makefile.kvm
> +++ b/tools/testing/selftests/kvm/Makefile.kvm
> @@ -303,10 +303,16 @@ arm64_hdr_outdir := $(tools_dir)/
>  endif
>  
>  GEN_HDRS := $(arm64_hdr_outdir)arch/arm64/include/generated/
> +GEN_SYSREG_DEFS := $(GEN_HDRS)asm/sysreg-defs.h
>  CFLAGS += -I$(GEN_HDRS)
>  
> -$(GEN_HDRS): $(wildcard $(arm64_tools_dir)/*)
> +# The inner Makefile tracks the generator's inputs and leaves the header
> +# untouched when nothing changed, so dependents rebuild only when it does.
> +$(GEN_SYSREG_DEFS): FORCE
>  	$(MAKE) -C $(arm64_tools_dir) OUTPUT=$(arm64_hdr_outdir)
> +
> +FORCE:
> +.PHONY: FORCE
>  endif

Definitely doesn't have to be now, but at some point should we split out some of
this arch-specific sub-Makefiles?  It's nice not having to hunt around to figure
out which tests are supported by which arch, but the amount of arch specific goo
we're accruing is getting unwieldy.

>  no-pie-option := $(call try-run, echo 'int main(void) { return 0; }' | \
> @@ -359,10 +365,10 @@ EXTRA_CLEAN += $(GEN_HDRS) \
>  	       $(TEST_GEN_OBJ) \
>  	       cscope.*
>  
> -$(LIBKVM_C_OBJ): $(OUTPUT)/%.o: %.c $(GEN_HDRS)
> +$(LIBKVM_C_OBJ): $(OUTPUT)/%.o: %.c $(GEN_SYSREG_DEFS)
>  	$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c $< -o $@
>  
> -$(LIBKVM_S_OBJ): $(OUTPUT)/%.o: %.S $(GEN_HDRS)
> +$(LIBKVM_S_OBJ): $(OUTPUT)/%.o: %.S $(GEN_SYSREG_DEFS)
>  	$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c $< -o $@
>  
>  # Compile the string overrides as freestanding to prevent the compiler from
> @@ -372,10 +378,10 @@ $(LIBKVM_STRING_OBJ): $(OUTPUT)/%.o: %.c
>  	$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c -ffreestanding $< -o $@
>  
>  $(shell mkdir -p $(sort $(dir $(TEST_GEN_PROGS))))
> -$(SPLIT_TEST_GEN_OBJ): $(GEN_HDRS)
> +$(SPLIT_TEST_GEN_OBJ): $(GEN_SYSREG_DEFS)
>  $(TEST_GEN_PROGS): $(LIBKVM_OBJS)
>  $(TEST_GEN_PROGS_EXTENDED): $(LIBKVM_OBJS)
> -$(TEST_GEN_OBJ): $(GEN_HDRS)
> +$(TEST_GEN_OBJ): $(GEN_SYSREG_DEFS)
>  
>  cscope: include_paths = $(LINUX_TOOL_INCLUDE) $(LINUX_HDR_PATH) include lib ..
>  cscope:
> -- 
> 2.39.5
>