[PATCH v3 1/4] perf build: Add ability to build with a generated vmlinux.h

[Ian Rogers <irogers@xxxxxxxxxx>]

Commit a887466562b4 ("perf bpf skels: Stop using vmlinux.h generated
from BTF, use subset of used structs + CO-RE") made it so that
vmlinux.h was uncondtionally included from
tools/perf/util/vmlinux.h. This change reverts part of that change (so
that vmlinux.h is once again generated) and makes it so that the
vmlinux.h used at build time is selected from the VMLINUX_H
variable. By default the VMLINUX_H variable is set to the vmlinux.h
added in change a887466562b4, but if GEN_VMLINUX_H=1 is passed on the
build command line then the previous generation behavior kicks in.

The build with GEN_VMLINUX_H=1 currently fails with:
```
util/bpf_skel/lock_contention.bpf.c:419:8: error: redefinition of 'rq'
struct rq {};
       ^
/tmp/perf/util/bpf_skel/.tmp/../vmlinux.h:45630:8: note: previous definition is here
struct rq {
       ^
1 error generated.
```

Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
 tools/perf/Makefile.config                       |  4 ++++
 tools/perf/Makefile.perf                         | 16 +++++++++++++++-
 tools/perf/util/bpf_skel/.gitignore              |  1 +
 tools/perf/util/bpf_skel/{ => vmlinux}/vmlinux.h |  0
 4 files changed, 20 insertions(+), 1 deletion(-)
 rename tools/perf/util/bpf_skel/{ => vmlinux}/vmlinux.h (100%)

diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
index a794d9eca93d..08d4e7eaa721 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -680,6 +680,10 @@ ifdef BUILD_BPF_SKEL
   CFLAGS += -DHAVE_BPF_SKEL
 endif
 
+ifndef GEN_VMLINUX_H
+  VMLINUX_H=$(src-perf)/util/bpf_skel/vmlinux/vmlinux.h
+endif
+
 dwarf-post-unwind := 1
 dwarf-post-unwind-text := BUG
 
diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index f48794816d82..f1840af195c0 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -1080,7 +1080,21 @@ $(BPFTOOL): | $(SKEL_TMP_OUT)
 	$(Q)CFLAGS= $(MAKE) -C ../bpf/bpftool \
 		OUTPUT=$(SKEL_TMP_OUT)/ bootstrap
 
-$(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) | $(SKEL_TMP_OUT)
+VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)				\
+		     $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)	\
+		     ../../vmlinux					\
+		     /sys/kernel/btf/vmlinux				\
+		     /boot/vmlinux-$(shell uname -r)
+VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
+
+$(SKEL_OUT)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL)
+ifeq ($(VMLINUX_H),)
+	$(QUIET_GEN)$(BPFTOOL) btf dump file $< format c > $@
+else
+	$(Q)cp "$(VMLINUX_H)" $@
+endif
+
+$(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
 	$(QUIET_CLANG)$(CLANG) -g -O2 -target bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
 	  -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@
 
diff --git a/tools/perf/util/bpf_skel/.gitignore b/tools/perf/util/bpf_skel/.gitignore
index 7a1c832825de..cd01455e1b53 100644
--- a/tools/perf/util/bpf_skel/.gitignore
+++ b/tools/perf/util/bpf_skel/.gitignore
@@ -1,3 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0-only
 .tmp
 *.skel.h
+vmlinux.h
diff --git a/tools/perf/util/bpf_skel/vmlinux.h b/tools/perf/util/bpf_skel/vmlinux/vmlinux.h
similarity index 100%
rename from tools/perf/util/bpf_skel/vmlinux.h
rename to tools/perf/util/bpf_skel/vmlinux/vmlinux.h
-- 
2.41.0.162.gfafddb0af9-goog