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: I035ad32233e49e2a30ce11f6c7c318a648511ef8

apex/dexpreopt_bootjars_test.go

@@ -164,6 +164,7 @@ func TestDexpreoptBootJarsWithSourceArtApex(t *testing.T) {
 		"out/soong/dexpreopt_arm64/dex_bootjars_input/baz.jar",
 		"out/soong/.intermediates/art-bootclasspath-fragment/android_common_apex10000/art-bootclasspath-fragment/boot.prof",
 		"out/soong/.intermediates/default/java/dex_bootjars/android_common/boot/boot.prof",
+		"out/soong/dexpreopt/uffd_gc_flag.txt",
 	}
 
 	expectedOutputs := []string{
@@ -201,6 +202,7 @@ func TestDexpreoptBootJarsWithPrebuiltArtApex(t *testing.T) {
 		"out/soong/dexpreopt_arm64/dex_bootjars_input/baz.jar",
 		"out/soong/.intermediates/prebuilt_com.android.art.deapexer/android_common/deapexer/etc/boot-image.prof",
 		"out/soong/.intermediates/default/java/dex_bootjars/android_common/boot/boot.prof",
+		"out/soong/dexpreopt/uffd_gc_flag.txt",
 	}
 
 	expectedOutputs := []string{

dexpreopt/config.go

@@ -98,7 +98,9 @@ type GlobalConfig struct {
 	// measure, as it masks real errors and affects performance.
 	RelaxUsesLibraryCheck bool
 
-	EnableUffdGc bool // preopt with the assumption that userfaultfd GC will be used on device.
+	// "true" to force preopt with CMC GC (a.k.a., UFFD GC); "false" to force preopt with CC GC;
+	// "default" to determine the GC type based on the kernel version file.
+	EnableUffdGc string
 }
 
 var allPlatformSystemServerJarsKey = android.NewOnceKey("allPlatformSystemServerJars")
@@ -154,6 +156,7 @@ type GlobalSoongConfig struct {
 	Zip2zip          android.Path
 	ManifestCheck    android.Path
 	ConstructContext android.Path
+	UffdGcFlag       android.WritablePath
 }
 
 type ModuleConfig struct {
@@ -537,6 +540,7 @@ func createGlobalSoongConfig(ctx android.ModuleContext) *GlobalSoongConfig {
 		Zip2zip:          ctx.Config().HostToolPath(ctx, "zip2zip"),
 		ManifestCheck:    ctx.Config().HostToolPath(ctx, "manifest_check"),
 		ConstructContext: ctx.Config().HostToolPath(ctx, "construct_context"),
+		UffdGcFlag:       getUffdGcFlagPath(ctx),
 	}
 }
 
@@ -588,6 +592,7 @@ type globalJsonSoongConfig struct {
 	Zip2zip          string
 	ManifestCheck    string
 	ConstructContext string
+	UffdGcFlag       string
 }
 
 // ParseGlobalSoongConfig parses the given data assumed to be read from the
@@ -609,6 +614,7 @@ func ParseGlobalSoongConfig(ctx android.PathContext, data []byte) (*GlobalSoongC
 		Zip2zip:          constructPath(ctx, jc.Zip2zip),
 		ManifestCheck:    constructPath(ctx, jc.ManifestCheck),
 		ConstructContext: constructPath(ctx, jc.ConstructContext),
+		UffdGcFlag:       constructWritablePath(ctx, jc.UffdGcFlag),
 	}
 
 	return config, nil
@@ -633,12 +639,15 @@ func checkBootJarsConfigConsistency(ctx android.SingletonContext, dexpreoptConfi
 }
 
 func (s *globalSoongConfigSingleton) GenerateBuildActions(ctx android.SingletonContext) {
-	checkBootJarsConfigConsistency(ctx, GetGlobalConfig(ctx), ctx.Config())
+	global := GetGlobalConfig(ctx)
+	checkBootJarsConfigConsistency(ctx, global, ctx.Config())
 
-	if GetGlobalConfig(ctx).DisablePreopt {
+	if global.DisablePreopt {
 		return
 	}
 
+	buildUffdGcFlag(ctx, global)
+
 	config := GetCachedGlobalSoongConfig(ctx)
 	if config == nil {
 		// No module has enabled dexpreopting, so we assume there will be no calls
@@ -654,6 +663,7 @@ func (s *globalSoongConfigSingleton) GenerateBuildActions(ctx android.SingletonC
 		Zip2zip:          config.Zip2zip.String(),
 		ManifestCheck:    config.ManifestCheck.String(),
 		ConstructContext: config.ConstructContext.String(),
+		UffdGcFlag:       config.UffdGcFlag.String(),
 	}
 
 	data, err := json.Marshal(jc)
@@ -684,9 +694,32 @@ func (s *globalSoongConfigSingleton) MakeVars(ctx android.MakeVarsContext) {
 		config.Zip2zip.String(),
 		config.ManifestCheck.String(),
 		config.ConstructContext.String(),
+		config.UffdGcFlag.String(),
 	}, " "))
 }
 
+func buildUffdGcFlag(ctx android.BuilderContext, global *GlobalConfig) {
+	uffdGcFlag := getUffdGcFlagPath(ctx)
+
+	if global.EnableUffdGc == "true" {
+		android.WriteFileRuleVerbatim(ctx, uffdGcFlag, "--runtime-arg -Xgc:CMC")
+	} else if global.EnableUffdGc == "false" {
+		android.WriteFileRuleVerbatim(ctx, uffdGcFlag, "")
+	} else if global.EnableUffdGc == "default" {
+		// Generated by `build/make/core/Makefile`.
+		kernelVersionFile := android.PathForOutput(ctx, "dexpreopt/kernel_version_for_uffd_gc.txt")
+		// Determine the UFFD GC flag by the kernel version file.
+		rule := android.NewRuleBuilder(pctx, ctx)
+		rule.Command().
+			Tool(ctx.Config().HostToolPath(ctx, "construct_uffd_gc_flag")).
+			Input(kernelVersionFile).
+			Output(uffdGcFlag)
+		rule.Restat().Build("dexpreopt_uffd_gc_flag", "dexpreopt_uffd_gc_flag")
+	} else {
+		panic(fmt.Sprintf("Unknown value of PRODUCT_ENABLE_UFFD_GC: %s", global.EnableUffdGc))
+	}
+}
+
 func GlobalConfigForTests(ctx android.PathContext) *GlobalConfig {
 	return &GlobalConfig{
 		DisablePreopt:                  false,
@@ -731,7 +764,7 @@ func GlobalConfigForTests(ctx android.PathContext) *GlobalConfig {
 	}
 }
 
-func globalSoongConfigForTests() *GlobalSoongConfig {
+func globalSoongConfigForTests(ctx android.BuilderContext) *GlobalSoongConfig {
 	return &GlobalSoongConfig{
 		Profman:          android.PathForTesting("profman"),
 		Dex2oat:          android.PathForTesting("dex2oat"),
@@ -740,5 +773,19 @@ func globalSoongConfigForTests() *GlobalSoongConfig {
 		Zip2zip:          android.PathForTesting("zip2zip"),
 		ManifestCheck:    android.PathForTesting("manifest_check"),
 		ConstructContext: android.PathForTesting("construct_context"),
+		UffdGcFlag:       android.PathForOutput(ctx, "dexpreopt_test", "uffd_gc_flag.txt"),
 	}
 }
+
+func GetDexpreoptDirName(ctx android.PathContext) string {
+	prefix := "dexpreopt_"
+	targets := ctx.Config().Targets[android.Android]
+	if len(targets) > 0 {
+		return prefix + targets[0].Arch.ArchType.String()
+	}
+	return prefix + "unknown_target"
+}
+
+func getUffdGcFlagPath(ctx android.PathContext) android.WritablePath {
+	return android.PathForOutput(ctx, "dexpreopt/uffd_gc_flag.txt")
+}

dexpreopt/dexpreopt.go

@@ -390,7 +390,8 @@ func dexpreoptCommand(ctx android.BuilderContext, globalSoong *GlobalSoongConfig
 		Flag("--generate-build-id").
 		Flag("--abort-on-hard-verifier-error").
 		Flag("--force-determinism").
-		FlagWithArg("--no-inline-from=", "core-oj.jar")
+		FlagWithArg("--no-inline-from=", "core-oj.jar").
+		Text("$(cat").Input(globalSoong.UffdGcFlag).Text(")")
 
 	var preoptFlags []string
 	if len(module.PreoptFlags) > 0 {
@@ -506,10 +507,6 @@ func dexpreoptCommand(ctx android.BuilderContext, globalSoong *GlobalSoongConfig
 		cmd.FlagWithInput("--profile-file=", profile)
 	}
 
-	if global.EnableUffdGc {
-		cmd.Flag("--runtime-arg").Flag("-Xgc:CMC")
-	}
-
 	rule.Install(odexPath, odexInstallPath)
 	rule.Install(vdexPath, vdexInstallPath)
 }

dexpreopt/dexpreopt_test.go

@@ -96,7 +96,7 @@ func createTestModuleConfig(name, dexLocation string, buildPath, dexPath, enforc
 func TestDexPreopt(t *testing.T) {
 	config := android.TestConfig("out", nil, "", nil)
 	ctx := android.BuilderContextForTesting(config)
-	globalSoong := globalSoongConfigForTests()
+	globalSoong := globalSoongConfigForTests(ctx)
 	global := GlobalConfigForTests(ctx)
 	module := testSystemModuleConfig(ctx, "test")
 	productPackages := android.PathForTesting("product_packages.txt")
@@ -114,12 +114,15 @@ func TestDexPreopt(t *testing.T) {
 	if rule.Installs().String() != wantInstalls.String() {
 		t.Errorf("\nwant installs:\n   %v\ngot:\n   %v", wantInstalls, rule.Installs())
 	}
+
+	android.AssertStringListContains(t, "", rule.Inputs().RelativeToTop().Strings(),
+		"out/soong/dexpreopt_test/uffd_gc_flag.txt")
 }
 
 func TestDexPreoptSystemOther(t *testing.T) {
 	config := android.TestConfig("out", nil, "", nil)
 	ctx := android.BuilderContextForTesting(config)
-	globalSoong := globalSoongConfigForTests()
+	globalSoong := globalSoongConfigForTests(ctx)
 	global := GlobalConfigForTests(ctx)
 	systemModule := testSystemModuleConfig(ctx, "Stest")
 	systemProductModule := testSystemProductModuleConfig(ctx, "SPtest")
@@ -180,7 +183,7 @@ func TestDexPreoptSystemOther(t *testing.T) {
 func TestDexPreoptApexSystemServerJars(t *testing.T) {
 	config := android.TestConfig("out", nil, "", nil)
 	ctx := android.BuilderContextForTesting(config)
-	globalSoong := globalSoongConfigForTests()
+	globalSoong := globalSoongConfigForTests(ctx)
 	global := GlobalConfigForTests(ctx)
 	module := testApexModuleConfig(ctx, "service-A", "com.android.apex1")
 	productPackages := android.PathForTesting("product_packages.txt")
@@ -204,7 +207,7 @@ func TestDexPreoptApexSystemServerJars(t *testing.T) {
 func TestDexPreoptStandaloneSystemServerJars(t *testing.T) {
 	config := android.TestConfig("out", nil, "", nil)
 	ctx := android.BuilderContextForTesting(config)
-	globalSoong := globalSoongConfigForTests()
+	globalSoong := globalSoongConfigForTests(ctx)
 	global := GlobalConfigForTests(ctx)
 	module := testPlatformSystemServerModuleConfig(ctx, "service-A")
 	productPackages := android.PathForTesting("product_packages.txt")
@@ -228,7 +231,7 @@ func TestDexPreoptStandaloneSystemServerJars(t *testing.T) {
 func TestDexPreoptSystemExtSystemServerJars(t *testing.T) {
 	config := android.TestConfig("out", nil, "", nil)
 	ctx := android.BuilderContextForTesting(config)
-	globalSoong := globalSoongConfigForTests()
+	globalSoong := globalSoongConfigForTests(ctx)
 	global := GlobalConfigForTests(ctx)
 	module := testSystemExtSystemServerModuleConfig(ctx, "service-A")
 	productPackages := android.PathForTesting("product_packages.txt")
@@ -252,7 +255,7 @@ func TestDexPreoptSystemExtSystemServerJars(t *testing.T) {
 func TestDexPreoptApexStandaloneSystemServerJars(t *testing.T) {
 	config := android.TestConfig("out", nil, "", nil)
 	ctx := android.BuilderContextForTesting(config)
-	globalSoong := globalSoongConfigForTests()
+	globalSoong := globalSoongConfigForTests(ctx)
 	global := GlobalConfigForTests(ctx)
 	module := testApexModuleConfig(ctx, "service-A", "com.android.apex1")
 	productPackages := android.PathForTesting("product_packages.txt")
@@ -276,7 +279,7 @@ func TestDexPreoptApexStandaloneSystemServerJars(t *testing.T) {
 func TestDexPreoptProfile(t *testing.T) {
 	config := android.TestConfig("out", nil, "", nil)
 	ctx := android.BuilderContextForTesting(config)
-	globalSoong := globalSoongConfigForTests()
+	globalSoong := globalSoongConfigForTests(ctx)
 	global := GlobalConfigForTests(ctx)
 	module := testSystemModuleConfig(ctx, "test")
 	productPackages := android.PathForTesting("product_packages.txt")
@@ -316,3 +319,55 @@ func TestDexPreoptConfigToJson(t *testing.T) {
 	after := fmt.Sprintf("%v", parsed)
 	android.AssertStringEquals(t, "The result must be the same as the original after marshalling and unmarshalling it.", before, after)
 }
+
+func TestUffdGcFlagForce(t *testing.T) {
+	for _, enableUffdGc := range []string{"true", "false"} {
+		t.Run(enableUffdGc, func(t *testing.T) {
+			preparers := android.GroupFixturePreparers(
+				PrepareForTestWithFakeDex2oatd,
+				PrepareForTestWithDexpreoptConfig,
+				FixtureSetEnableUffdGc(enableUffdGc),
+			)
+
+			result := preparers.RunTest(t)
+			ctx := result.TestContext
+
+			ctx.SingletonForTests("dexpreopt-soong-config").Output("out/soong/dexpreopt/uffd_gc_flag.txt")
+		})
+	}
+}
+
+func TestUffdGcFlagDefault(t *testing.T) {
+	preparers := android.GroupFixturePreparers(
+		PrepareForTestWithFakeDex2oatd,
+		PrepareForTestWithDexpreoptConfig,
+		FixtureSetEnableUffdGc("default"),
+	)
+
+	result := preparers.RunTest(t)
+	ctx := result.TestContext
+	config := ctx.Config()
+
+	rule := ctx.SingletonForTests("dexpreopt-soong-config").Rule("dexpreopt_uffd_gc_flag")
+
+	android.AssertStringDoesContain(t, "", rule.RuleParams.Command, "construct_uffd_gc_flag")
+	android.AssertStringPathsRelativeToTopEquals(t, "", config, []string{
+		"out/soong/dexpreopt/uffd_gc_flag.txt",
+	}, rule.AllOutputs())
+	android.AssertPathsRelativeToTopEquals(t, "", []string{
+		"out/soong/dexpreopt/kernel_version_for_uffd_gc.txt",
+	}, rule.Implicits)
+}
+
+func TestUffdGcFlagBogus(t *testing.T) {
+	preparers := android.GroupFixturePreparers(
+		PrepareForTestWithFakeDex2oatd,
+		PrepareForTestWithDexpreoptConfig,
+		FixtureSetEnableUffdGc("bogus"),
+	)
+
+	preparers.
+		ExtendWithErrorHandler(android.FixtureExpectsAtLeastOneErrorMatchingPattern(
+			"Unknown value of PRODUCT_ENABLE_UFFD_GC: bogus")).
+		RunTest(t)
+}

dexpreopt/testing.go

@@ -88,6 +88,15 @@ var PrepareForTestByEnablingDexpreopt = android.GroupFixturePreparers(
 	FixtureModifyGlobalConfig(func(android.PathContext, *GlobalConfig) {}),
 )
 
+var PrepareForTestWithDexpreoptConfig = android.GroupFixturePreparers(
+	android.PrepareForTestWithAndroidBuildComponents,
+	android.FixtureModifyContext(func(ctx *android.TestContext) {
+		ctx.RegisterParallelSingletonType("dexpreopt-soong-config", func() android.Singleton {
+			return &globalSoongConfigSingleton{}
+		})
+	}),
+)
+
 // FixtureModifyGlobalConfig enables dexpreopt (unless modified by the mutator) and modifies the
 // configuration.
 func FixtureModifyGlobalConfig(configModifier func(ctx android.PathContext, dexpreoptConfig *GlobalConfig)) android.FixturePreparer {
@@ -195,3 +204,10 @@ func FixtureDisableDexpreopt(disable bool) android.FixturePreparer {
 		dexpreoptConfig.DisablePreopt = disable
 	})
 }
+
+// FixtureSetEnableUffdGc sets the EnableUffdGc property in the global config.
+func FixtureSetEnableUffdGc(value string) android.FixturePreparer {
+	return FixtureModifyGlobalConfig(func(_ android.PathContext, dexpreoptConfig *GlobalConfig) {
+		dexpreoptConfig.EnableUffdGc = value
+	})
+}

java/dexpreopt_bootjars.go

@@ -617,7 +617,8 @@ func (d *dexpreoptBootJars) GenerateAndroidBuildActions(ctx android.ModuleContex
 
 // GenerateSingletonBuildActions generates build rules for the dexpreopt config for Make.
 func (d *dexpreoptBootJars) GenerateSingletonBuildActions(ctx android.SingletonContext) {
-	d.dexpreoptConfigForMake = android.PathForOutput(ctx, getDexpreoptDirName(ctx), "dexpreopt.config")
+	d.dexpreoptConfigForMake =
+		android.PathForOutput(ctx, dexpreopt.GetDexpreoptDirName(ctx), "dexpreopt.config")
 	writeGlobalConfigForMake(ctx, d.dexpreoptConfigForMake)
 }
 
@@ -1066,8 +1067,8 @@ func buildBootImageVariant(ctx android.ModuleContext, image *bootImageVariant, p
 		cmd.FlagWithArg("--instruction-set-features=", global.InstructionSetFeatures[arch])
 	}
 
-	if global.EnableUffdGc && image.target.Os == android.Android {
+	if image.target.Os == android.Android {
-		cmd.Flag("--runtime-arg").Flag("-Xgc:CMC")
+		cmd.Text("$(cat").Input(globalSoong.UffdGcFlag).Text(")")
 	}
 
 	if global.BootFlags != "" {
@@ -1235,7 +1236,7 @@ func bootFrameworkProfileRule(ctx android.ModuleContext, image *bootImageConfig)
 func dumpOatRules(ctx android.ModuleContext, image *bootImageConfig) {
 	var allPhonies android.Paths
 	name := image.name
-	global := dexpreopt.GetGlobalConfig(ctx)
+	globalSoong := dexpreopt.GetGlobalSoongConfig(ctx)
 	for _, image := range image.variants {
 		arch := image.target.Arch.ArchType
 		suffix := arch.String()
@@ -1247,6 +1248,7 @@ func dumpOatRules(ctx android.ModuleContext, image *bootImageConfig) {
 		output := android.PathForOutput(ctx, name+"."+suffix+".oatdump.txt")
 		rule := android.NewRuleBuilder(pctx, ctx)
 		imageLocationsOnHost, _ := image.imageLocations()
+
 		cmd := rule.Command().
 			BuiltTool("oatdump").
 			FlagWithInputList("--runtime-arg -Xbootclasspath:", image.dexPathsDeps.Paths(), ":").
@@ -1254,8 +1256,8 @@ func dumpOatRules(ctx android.ModuleContext, image *bootImageConfig) {
 			FlagWithArg("--image=", strings.Join(imageLocationsOnHost, ":")).Implicits(image.imagesDeps.Paths()).
 			FlagWithOutput("--output=", output).
 			FlagWithArg("--instruction-set=", arch.String())
-		if global.EnableUffdGc && image.target.Os == android.Android {
+		if image.target.Os == android.Android {
-			cmd.Flag("--runtime-arg").Flag("-Xgc:CMC")
+			cmd.Text("$(cat").Input(globalSoong.UffdGcFlag).Text(")")
 		}
 		rule.Build("dump-oat-"+name+"-"+suffix, "dump oat "+name+" "+arch.String())
 

java/dexpreopt_config.go

@@ -115,7 +115,7 @@ func genBootImageConfigRaw(ctx android.PathContext) map[string]*bootImageConfig
 func genBootImageConfigs(ctx android.PathContext) map[string]*bootImageConfig {
 	return ctx.Config().Once(bootImageConfigKey, func() interface{} {
 		targets := dexpreoptTargets(ctx)
-		deviceDir := android.PathForOutput(ctx, getDexpreoptDirName(ctx))
+		deviceDir := android.PathForOutput(ctx, dexpreopt.GetDexpreoptDirName(ctx))
 
 		configs := genBootImageConfigRaw(ctx)
 
@@ -234,12 +234,3 @@ func init() {
 func dexpreoptConfigMakevars(ctx android.MakeVarsContext) {
 	ctx.Strict("DEXPREOPT_BOOT_JARS_MODULES", strings.Join(defaultBootImageConfig(ctx).modules.CopyOfApexJarPairs(), ":"))
 }
-
-func getDexpreoptDirName(ctx android.PathContext) string {
-	prefix := "dexpreopt_"
-	targets := ctx.Config().Targets[android.Android]
-	if len(targets) > 0 {
-		return prefix + targets[0].Arch.ArchType.String()
-	}
-	return prefix + "unknown_target"
-}

scripts/Android.bp

@@ -142,6 +142,39 @@ python_test_host {
     test_suites: ["general-tests"],
 }
 
+python_library_host {
+    name: "uffd_gc_utils",
+    srcs: [
+        "uffd_gc_utils.py",
+    ],
+    visibility: [
+        "//build/make/tools:__subpackages__",
+    ],
+}
+
+python_test_host {
+    name: "uffd_gc_utils_test",
+    main: "uffd_gc_utils_test.py",
+    srcs: [
+        "uffd_gc_utils_test.py",
+    ],
+    libs: [
+        "uffd_gc_utils",
+    ],
+    test_suites: ["general-tests"],
+}
+
+python_binary_host {
+    name: "construct_uffd_gc_flag",
+    main: "construct_uffd_gc_flag.py",
+    srcs: [
+        "construct_uffd_gc_flag.py",
+    ],
+    libs: [
+        "uffd_gc_utils",
+    ],
+}
+
 python_library_host {
     name: "ninja_rsp",
     srcs: ["ninja_rsp.py"],

scripts/construct_uffd_gc_flag.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2024 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+"""A tool for constructing UFFD GC flag."""
+
+import argparse
+import os
+
+from uffd_gc_utils import should_enable_uffd_gc
+
+
+def parse_args():
+  parser = argparse.ArgumentParser()
+  parser.add_argument('kernel_version_file')
+  parser.add_argument('output')
+  return parser.parse_args()
+
+def main():
+  args = parse_args()
+  enable_uffd_gc = should_enable_uffd_gc(args.kernel_version_file)
+  flag = '--runtime-arg -Xgc:CMC' if enable_uffd_gc else ''
+  # Prevent the file's mtime from being changed if the contents don't change.
+  # This avoids unnecessary dexpreopt reruns.
+  if os.path.isfile(args.output):
+    with open(args.output, 'r') as f:
+      if f.read() == flag:
+        return
+  with open(args.output, 'w') as f:
+    f.write(flag)
+
+
+if __name__ == '__main__':
+  main()

scripts/uffd_gc_utils.py

@@ -0,0 +1,68 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2024 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+"""Utils to determine whether to enable UFFD GC."""
+
+import re
+import sys
+
+
+def should_enable_uffd_gc(kernel_version_file):
+  with open(kernel_version_file, 'r') as f:
+    kernel_version = f.read().strip()
+  return should_enable_uffd_gc_impl(kernel_version)
+
+def should_enable_uffd_gc_impl(kernel_version):
+  # See https://source.android.com/docs/core/architecture/kernel/gki-versioning#determine-release
+  p = r"^(?P<w>\d+)[.](?P<x>\d+)[.](?P<y>\d+)(-android(?P<z>\d+)-(?P<k>\d+).*$)?"
+  m = re.match(p, kernel_version)
+  if m is not None:
+    if m.group('z') is not None:
+      android_release = int(m.group('z'))
+      # No need to check w, x, y because all Android 12 kernels have backports.
+      return android_release >= 12
+    else:
+      # Old kernel or non-GKI kernel.
+      version = int(m.group('w'))
+      patch_level = int(m.group('x'))
+      if version < 5:
+        # Old kernel.
+        return False
+      elif (version == 5 and patch_level >= 7) or version >= 6:
+        # New non-GKI kernel. 5.7 supports MREMAP_DONTUNMAP without the need for
+        # backports.
+        return True
+      else:
+        # Non-GKI kernel between 5 and 5.6. It may have backports.
+        raise exit_with_error(kernel_version)
+  elif kernel_version == '<unknown-kernel>':
+    # The kernel information isn't available to the build system, probably
+    # because PRODUCT_OTA_ENFORCE_VINTF_KERNEL_REQUIREMENTS is set to false. We
+    # assume that the kernel supports UFFD GC because it is the case for most of
+    # the products today and it is the future.
+    return True
+  else:
+    # Unrecognizable non-GKI kernel.
+    raise exit_with_error(kernel_version)
+
+def exit_with_error(kernel_version):
+  sys.exit(f"""
+Unable to determine UFFD GC flag for kernel version "{kernel_version}".
+You can fix this by explicitly setting PRODUCT_ENABLE_UFFD_GC to "true" or
+"false" based on the kernel version.
+1. Set PRODUCT_ENABLE_UFFD_GC to "true" if the kernel supports userfaultfd(2)
+   and MREMAP_DONTUNMAP.
+2. Set PRODUCT_ENABLE_UFFD_GC to "false" otherwise.""")

scripts/uffd_gc_utils_test.py

@@ -0,0 +1,61 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2024 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+"""Unit tests for uffd_gc_utils.py."""
+
+import unittest
+
+from uffd_gc_utils import should_enable_uffd_gc_impl
+
+
+class UffdGcUtilsTest(unittest.TestCase):
+  def test_should_enable_uffd_gc_impl(self):
+    # GKI kernels in new format.
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "6.1.25-android14-11-g34fde9ec08a3-ab10675345"))
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "5.4.42-android12-0-something"))
+    self.assertFalse(should_enable_uffd_gc_impl(
+        "5.4.42-android11-0-something"))
+    # GKI kernels in old format.
+    self.assertFalse(should_enable_uffd_gc_impl(
+        "4.19.282-g4b749a433956-ab10893502"))
+    # Non GKI kernels.
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "6.1.25-foo"))
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "6.1.25"))
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "5.10.19-foo"))
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "5.10.19"))
+    with self.assertRaises(SystemExit):
+        should_enable_uffd_gc_impl("5.4.42-foo")
+    with self.assertRaises(SystemExit):
+        should_enable_uffd_gc_impl("5.4.42")
+    self.assertFalse(should_enable_uffd_gc_impl(
+        "4.19.282-foo"))
+    self.assertFalse(should_enable_uffd_gc_impl(
+        "4.19.282"))
+    with self.assertRaises(SystemExit):
+        should_enable_uffd_gc_impl("foo")
+    # No kernel.
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "<unknown-kernel>"))
+
+
+if __name__ == '__main__':
+  unittest.main(verbosity=2)