Determine GC type based on BUILT_KERNEL_VERSION_FILE.

How it works:
1. build/make/core/Makefile generates a txt file with the kernel
   version, which is taken from an explicit BOARD_KERNEL_VERSION value,
   or extracted from the kernel image on the source tree, or extracted
   from the kernel image extracted from the prebuilt boot.img.
   The file is saved at
   $ANDROID_PRODUCT_OUT/obj/PACKAGING/check_vintf_all_intermediates/kernel_version.txt.
2. If PRODUCT_ENABLE_UFFD_GC is "default", meaning the GC type needs to
   be determined by the kernel version, build/make/core/Makefile copies
   kernel_version.txt to
   out/soong/dexpreopt/kernel_version_for_uffd_gc.txt.
3. build/soong/dexpreopt/config.go writes the the UFFD GC flag to
   out/soong/dexpreopt/uffd_gc_flag.txt. The flag is determined by an
   explicit PRODUCT_ENABLE_UFFD_GC value or by contruct_uffd_gc_flag.py,
   which reads kernel_version_for_uffd_gc.txt and determines the flag
   accordingly.
4. dex2oat takes the UFFD GC flag from uffd_gc_flag.txt.
5. post_process_props.py mangles ro.dalvik.vm.enable_uffd_gc based on
   the same logic.

Bug: 321751629
Bug: 319554951
Bug: 318763448
Bug: 319648491
Test: m --no-skip-soong-tests nothing
Test: atest uffd_gc_utils_test
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=default m` for device with no
  UFFD support -
  1. Check the existence of `-Xgc:CMC` in
     out/soong/dexpreopt_arm64/dex_bootjars/android/system/framework/arm64/boot.invocation
     (dex2oat invocation for a boot image)
  2. Check the existence of `-Xgc:CMC` in
     out/soong/.intermediates/packages/apps/Settings/Settings/android_common/dexpreopt/Settings/oat/arm64/package.invocation
     (dex2oat invocation for an app defined in .bp)
  3. Check the existence of `-Xgc:CMC` in
     $ANDROID_PRODUCT_OUT/obj/APPS/Dialer_intermediates/oat/arm64/package.invocation
     (dex2oat invocation for an app defined in .mk)
  4. Check the value of ro.dalvik.vm.enable_uffd_gc in
     $ANDROID_PRODUCT_OUT/product/etc/build.prop
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=default m` for device with
  UFFD support, and do the steps above.
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=true m`, and do the steps
  above.
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=false m`, and do the steps
  above.

Change-Id: I8df6e5be1644da05d2d5c57b3520f29601dfd7a4
diff --git a/core/Makefile b/core/Makefile
index b3870e5..94fc88e 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -287,6 +287,11 @@
 endif
 endif
 
+# Do this early because sysprop.mk depends on BUILT_KERNEL_VERSION_FILE_FOR_UFFD_GC.
+ifeq (default,$(ENABLE_UFFD_GC))
+BUILT_KERNEL_VERSION_FILE_FOR_UFFD_GC := $(OUT_DIR)/soong/dexpreopt/kernel_version_for_uffd_gc.txt
+endif # ENABLE_UFFD_GC
+
 include $(BUILD_SYSTEM)/sysprop.mk
 
 # ----------------------------------------------------------------
@@ -5256,6 +5261,34 @@
 
 endif # PRODUCT_OTA_ENFORCE_VINTF_KERNEL_REQUIREMENTS
 
+ifeq (default,$(ENABLE_UFFD_GC))
+
+ifneq (,$(BUILT_KERNEL_VERSION_FILE))
+$(BUILT_KERNEL_VERSION_FILE_FOR_UFFD_GC): $(BUILT_KERNEL_VERSION_FILE)
+$(BUILT_KERNEL_VERSION_FILE_FOR_UFFD_GC):
+	cp $(BUILT_KERNEL_VERSION_FILE) $(BUILT_KERNEL_VERSION_FILE_FOR_UFFD_GC)
+else
+# We make this a warning rather than an error to avoid breaking too many builds. When it happens,
+# we use a placeholder as the kernel version, which is consumed by uffd_gc_utils.py.
+$(BUILT_KERNEL_VERSION_FILE_FOR_UFFD_GC):
+	echo $$'\
+Unable to determine UFFD GC flag because the kernel version is not available and\n\
+PRODUCT_ENABLE_UFFD_GC is "default".\n\
+You can fix this by:\n\
+  1. [Recommended] Making the kernel version available.\n\
+    (1). Set PRODUCT_OTA_ENFORCE_VINTF_KERNEL_REQUIREMENTS to "true".\n\
+    (2). If you are still getting this message after doing so, see the warning about\n\
+         PRODUCT_OTA_ENFORCE_VINTF_KERNEL_REQUIREMENTS in the build logs.\n\
+  or\n\
+  2. Explicitly setting PRODUCT_ENABLE_UFFD_GC to "true" or "false" based on the kernel version.\n\
+    (1). Set PRODUCT_ENABLE_UFFD_GC to "true" if the kernel is a GKI kernel and is android12-5.4\n\
+         or above, or a non-GKI kernel that supports userfaultfd(2) and MREMAP_DONTUNMAP.\n\
+    (2). Set PRODUCT_ENABLE_UFFD_GC to "false" otherwise.'\
+	  && echo '<unknown-kernel>' > $@
+endif # BUILT_KERNEL_VERSION_FILE
+
+endif # ENABLE_UFFD_GC == "default"
+
 # -- Check VINTF compatibility of build.
 # Skip partial builds; only check full builds. Only check if:
 # - PRODUCT_ENFORCE_VINTF_MANIFEST is true