Bug 1661016: aarch64: Invalidate icache when compiling on a background thread; r=nbp,lth

On real hardware, when a background thread finishes compilation, it must signal
to the other executing threads that they need to reload a new version of the
code. Ideally, each executing thread would run an ISB instruction to do so. We
hereby use a system call membarrier that interrupts every other running thread,
and will cause the same effect as a local ISB would. It is heavyweight, so we
make sure to only run it in the case where we're on a background thread.

In the simulator, pending icache flushing requests were never satisfied before
this patch, when the request was emitted from a thread other than the main
thread. Similar behavior as above is emulated.

Differential Revision: https://phabricator.services.mozilla.com/D88395

js/src/jit/ExecutableAllocator.h

@@ -177,9 +177,19 @@ class ExecutableAllocator {
   }
 
   MOZ_MUST_USE
-  static bool makeExecutableAndFlushICache(void* start, size_t size) {
+  static bool makeExecutableAndFlushICache(FlushICacheSpec flushSpec,
+                                           void* start, size_t size) {
+    MustFlushICache mustFlushICache;
+    switch (flushSpec) {
+      case FlushICacheSpec::LocalThreadOnly:
+        mustFlushICache = MustFlushICache::LocalThreadOnly;
+        break;
+      case FlushICacheSpec::AllThreads:
+        mustFlushICache = MustFlushICache::AllThreads;
+        break;
+    }
     return ReprotectRegion(start, size, ProtectionSetting::Executable,
-                           MustFlushICache::Yes);
+                           mustFlushICache);
   }
 
   static void poisonCode(JSRuntime* rt, JitPoisonRangeVector& ranges);

js/src/jit/FlushICache.h

@@ -18,18 +18,22 @@ namespace jit {
 
 #if defined(JS_CODEGEN_X86) || defined(JS_CODEGEN_X64)
 
-inline void FlushICache(void* code, size_t size) {
+inline void FlushICache(void* code, size_t size,
+                        bool codeIsThreadLocal = true) {
   // No-op. Code and data caches are coherent on x86 and x64.
 }
 
 #elif (defined(JS_CODEGEN_ARM) || defined(JS_CODEGEN_ARM64)) || \
     (defined(JS_CODEGEN_MIPS32) || defined(JS_CODEGEN_MIPS64))
 
-extern void FlushICache(void* code, size_t size);
+extern void FlushICache(void* code, size_t size, bool codeIsThreadLocal = true);
 
 #elif defined(JS_CODEGEN_NONE)
 
-inline void FlushICache(void* code, size_t size) { MOZ_CRASH(); }
+inline void FlushICache(void* code, size_t size,
+                        bool codeIsThreadLocal = true) {
+  MOZ_CRASH();
+}
 
 #else
 #  error "Unknown architecture!"

js/src/jit/JitRealm.h

@@ -742,7 +742,8 @@ class MOZ_RAII AutoWritableJitCodeFallible {
   }
 
   ~AutoWritableJitCodeFallible() {
-    if (!ExecutableAllocator::makeExecutableAndFlushICache(addr_, size_)) {
+    if (!ExecutableAllocator::makeExecutableAndFlushICache(
+            FlushICacheSpec::LocalThreadOnly, addr_, size_)) {
       MOZ_CRASH();
     }
     rt_->toggleAutoWritableJitCodeActive(false);

js/src/jit/ProcessExecutableMemory.cpp

@@ -738,9 +738,11 @@ bool js::jit::ReprotectRegion(void* start, size_t size,
                               ProtectionSetting protection,
                               MustFlushICache flushICache) {
   // Flush ICache when making code executable, before we modify |size|.
-  if (flushICache == MustFlushICache::Yes) {
+  if (flushICache == MustFlushICache::LocalThreadOnly ||
+      flushICache == MustFlushICache::AllThreads) {
     MOZ_ASSERT(protection == ProtectionSetting::Executable);
-    jit::FlushICache(start, size);
+    bool codeIsThreadLocal = flushICache == MustFlushICache::LocalThreadOnly;
+    jit::FlushICache(start, size, codeIsThreadLocal);
   }
 
   // Calculate the start of the page containing this region,

js/src/jit/ProcessExecutableMemory.h

@@ -70,7 +70,15 @@ enum class ProtectionSetting {
   Executable,
 };
 
-enum class MustFlushICache { No, Yes };
+/// Whether the instruction cache must be flushed:
+//- No means no flushing will happen.
+//- LocalThreadOnly means only the local thread's icache will be flushed.
+//- AllThreads means all the threads' icaches will be flushed; this must be used
+// when the compiling thread and the executing thread might be different.
+
+enum class MustFlushICache { No, LocalThreadOnly, AllThreads };
+
+enum class FlushICacheSpec { LocalThreadOnly, AllThreads };
 
 extern MOZ_MUST_USE bool ReprotectRegion(void* start, size_t size,
                                          ProtectionSetting protection,

js/src/jit/arm/Architecture-arm.cpp

@@ -452,7 +452,7 @@ uint32_t FloatRegisters::ActualTotalPhys() {
   return 16;
 }
 
-void FlushICache(void* code, size_t size) {
+void FlushICache(void* code, size_t size, bool codeIsThreadLocal) {
 #if defined(JS_SIMULATOR_ARM)
   js::jit::SimulatorProcess::FlushICache(code, size);
 

js/src/jit/arm64/Architecture-arm64.cpp

@@ -78,8 +78,12 @@ uint32_t FloatRegister::getRegisterDumpOffsetInBytes() {
 
 uint32_t GetARM64Flags() { return 0; }
 
-void FlushICache(void* code, size_t size) {
-  vixl::CPU::EnsureIAndDCacheCoherency(code, size);
+void FlushICache(void* code, size_t size, bool codeIsThreadLocal) {
+  vixl::CPU::EnsureIAndDCacheCoherency(code, size, codeIsThreadLocal);
+}
+
+bool CanFlushICacheFromBackgroundThreads() {
+  return vixl::CPU::CanFlushICacheFromBackgroundThreads();
 }
 
 }  // namespace jit

js/src/jit/arm64/Architecture-arm64.h

@@ -616,6 +616,8 @@ inline bool hasMultiAlias() { return false; }
 
 uint32_t GetARM64Flags();
 
+bool CanFlushICacheFromBackgroundThreads();
+
 }  // namespace jit
 }  // namespace js
 

js/src/jit/arm64/vixl/Cpu-vixl.h

@@ -165,7 +165,11 @@ class CPU {
   // the I and D caches. I and D caches are not automatically coherent on ARM
   // so this operation is required before any dynamically generated code can
   // safely run.
-  static void EnsureIAndDCacheCoherency(void *address, size_t length);
+  static void EnsureIAndDCacheCoherency(void *address, size_t length, bool codeIsThreadLocal);
+
+  // Returns true when the current machine supports flushing the instruction
+  // cache on a background thread.
+  static bool CanFlushICacheFromBackgroundThreads();
 
   // Read and interpret the ID registers. This requires
   // CPUFeatures::kIDRegisterEmulation, and therefore cannot be called on

js/src/jit/arm64/vixl/MozCpu-vixl.cpp

@@ -33,6 +33,26 @@
 #  include <libkern/OSCacheControl.h>
 #endif
 
+#if defined(__aarch64__) && !defined(_MSC_VER) && !defined(XP_DARWIN)
+#   if defined(__linux__)
+#    include <linux/membarrier.h>
+#    include <sys/syscall.h>
+#    include <sys/utsname.h>
+#   elif defined(__ANDROID__)
+#    include <sys/syscall.h>
+#    include <unistd.h>
+#   else
+#    error "Missing platform-specific declarations for membarrier syscall!"
+#   endif // __linux__ / ANDROID
+
+#  include "vm/JSContext.h" // TlsContext
+
+static int membarrier(int cmd, int flags) {
+    return syscall(__NR_membarrier, cmd, flags);
+}
+
+#endif // __aarch64__
+
 namespace vixl {
 
 
@@ -84,8 +104,47 @@ uint32_t CPU::GetCacheType() {
 #endif
 }
 
+bool CPU::CanFlushICacheFromBackgroundThreads() {
+#if defined(__aarch64__) && !defined(_MSC_VER) && !defined(XP_DARWIN)
+  // On linux, check the kernel supports membarrier(2), that is, it's a kernel
+  // above Linux 4.16 included.
+  //
+  // Note: this code has been extracted (August 2020) from
+  // https://android.googlesource.com/platform/art/+/58520dfba31d6eeef75f5babff15e09aa28e5db8/libartbase/base/membarrier.cc#50
+  static constexpr int kRequiredMajor = 4;
+  static constexpr int kRequiredMinor = 16;
 
-void CPU::EnsureIAndDCacheCoherency(void *address, size_t length) {
+  static bool computed = false;
+  static bool kernelHasMembarrier = false;
+
+  if (!computed) {
+    struct utsname uts;
+    int major, minor;
+    kernelHasMembarrier = uname(&uts) == 0 &&
+        strcmp(uts.sysname, "Linux") == 0 &&
+        sscanf(uts.release, "%d.%d", &major, &minor) == 2 &&
+        major >= kRequiredMajor && (major != kRequiredMajor || minor >= kRequiredMinor);
+    computed = true;
+  }
+
+  if (!kernelHasMembarrier) {
+    return false;
+  }
+
+  // As a test bed, try to run the syscall with the command registering the
+  // intent to use the actual membarrier we'll want to carry out later.
+  if (membarrier(MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE, 0) != 0) {
+    return false;
+  }
+
+  return true;
+#else
+  // On other platforms, we assume that the provided syscall does the right thing.
+  return true;
+#endif
+}
+
+void CPU::EnsureIAndDCacheCoherency(void *address, size_t length, bool codeIsThreadLocal) {
 #if defined(JS_SIMULATOR_ARM64) && defined(JS_CACHE_SIMULATOR_ARM64)
   // This code attempts to emulate what the following assembly sequence is
   // doing, which is sending the information to all cores that some cache line
@@ -107,6 +166,11 @@ void CPU::EnsureIAndDCacheCoherency(void *address, size_t length) {
   Simulator* sim = vixl::Simulator::Current();
   if (sim) {
     sim->FlushICache();
+  } else if (!codeIsThreadLocal) {
+    // We're on a background thread; emulate what the real hardware would do by
+    // emitting a membarrier that'll interrupt and cause an icache invalidation
+    // on all the threads.
+    SimulatorProcess::membarrier();
   }
 #elif defined(_MSC_VER) && defined(_M_ARM64)
   FlushInstructionCache(GetCurrentProcess(), address, length);
@@ -199,6 +263,21 @@ void CPU::EnsureIAndDCacheCoherency(void *address, size_t length) {
     // isb : Instruction Synchronisation Barrier
     "   isb\n"
     : : : "memory");
+
+  if (!codeIsThreadLocal) {
+    // If we're on a background thread, emit a membarrier that will synchronize
+    // all the executing threads with the new version of the code.
+    JSContext* cx = js::TlsContext.get();
+    if (!cx || !cx->isMainThreadContext()) {
+      MOZ_RELEASE_ASSERT(CPU::CanFlushICacheFromBackgroundThreads());
+      // The intent to use this command has been carried over in
+      // CanFlushICacheFromBackgroundThreads.
+      if (membarrier(MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE, 0) != 0) {
+        // Better safe than sorry.
+        MOZ_CRASH("membarrier can't be executed");
+      }
+    }
+  }
 #else
   // If the host isn't AArch64, we must be using the simulator, so this function
   // doesn't have to do anything.

js/src/jit/arm64/vixl/MozSimulator-vixl.cpp

@@ -208,6 +208,16 @@ void Simulator::Destroy(Simulator* sim) {
 void Simulator::ExecuteInstruction() {
   // The program counter should always be aligned.
   VIXL_ASSERT(IsWordAligned(pc_));
+  if (pendingCacheRequests) {
+      // We're here emulating the behavior of the membarrier carried over on
+      // real hardware does; see syscalls to membarrier in MozCpu-vixl.cpp.
+      // There's a slight difference that the simulator is not being
+      // interrupted: instead, we effectively run the icache flush request
+      // before executing the next instruction, which is close enough and
+      // sufficient for our use case.
+      js::jit::AutoLockSimulatorCache alsc;
+      FlushICache();
+  }
   decoder_->Decode(pc_);
   increment_pc();
 }
@@ -889,6 +899,7 @@ Simulator::FlushICache()
     decoder_->FlushICache(flush.start, flush.length);
   }
   vec.clear();
+  pendingCacheRequests = false;
 }
 
 void CachingDecoder::Decode(const Instruction* instr) {
@@ -968,6 +979,13 @@ void SimulatorProcess::recordICacheFlush(void* start, size_t length) {
   }
 }
 
+void SimulatorProcess::membarrier() {
+  MOZ_ASSERT(singleton_->lock_.ownedByCurrentThread());
+  for (auto& s : singleton_->pendingFlushes_) {
+    s.thread->pendingCacheRequests = true;
+  }
+}
+
 SimulatorProcess::ICacheFlushes& SimulatorProcess::getICacheFlushes(Simulator* sim) {
   MOZ_ASSERT(singleton_->lock_.ownedByCurrentThread());
   for (auto& s : singleton_->pendingFlushes_) {

js/src/jit/arm64/vixl/Simulator-vixl.h

@@ -503,6 +503,7 @@ class Simulator : public DecoderVisitor {
  public:
 #ifdef JS_CACHE_SIMULATOR_ARM64
   using Decoder = CachingDecoder;
+  mozilla::Atomic<bool> pendingCacheRequests = mozilla::Atomic<bool>{ false };
 #endif
   explicit Simulator(Decoder* decoder, FILE* stream = stdout);
   ~Simulator();
@@ -2522,6 +2523,7 @@ class SimulatorProcess
   mozilla::Vector<SimFlushes, 1> pendingFlushes_;
 
   static void recordICacheFlush(void* start, size_t length);
+  static void membarrier();
   static ICacheFlushes& getICacheFlushes(Simulator* sim);
   static MOZ_MUST_USE bool registerSimulator(Simulator* sim);
   static void unregisterSimulator(Simulator* sim);

js/src/jit/mips-shared/Architecture-mips-shared.cpp

@@ -82,7 +82,7 @@ Registers::Code Registers::FromName(const char* name) {
   return Invalid;
 }
 
-void FlushICache(void* code, size_t size) {
+void FlushICache(void* code, size_t size, bool codeIsThreadLocal) {
 #if defined(JS_SIMULATOR)
   js::jit::SimulatorProcess::FlushICache(code, size);
 

js/src/jit/shared/AtomicOperations-shared-jit.cpp

@@ -876,8 +876,8 @@ bool InitializeJittedAtomics() {
   masm.executableCopy(code);
 
   // Reprotect the whole region to avoid having separate RW and RX mappings.
-  if (!ExecutableAllocator::makeExecutableAndFlushICache(code,
-                                                         roundedCodeLength)) {
+  if (!ExecutableAllocator::makeExecutableAndFlushICache(
+          FlushICacheSpec::LocalThreadOnly, code, roundedCodeLength)) {
     DeallocateExecutableMemory(code, roundedCodeLength);
     return false;
   }

js/src/jsapi-tests/testJitMacroAssembler.cpp

@@ -49,8 +49,8 @@ static bool Execute(JSContext* cx, MacroAssembler& masm) {
   if (!code) {
     return false;
   }
-  if (!ExecutableAllocator::makeExecutableAndFlushICache(code->raw(),
-                                                         code->bufferSize())) {
+  if (!ExecutableAllocator::makeExecutableAndFlushICache(
+          FlushICacheSpec::LocalThreadOnly, code->raw(), code->bufferSize())) {
     return false;
   }
 

js/src/wasm/WasmBuiltins.cpp

@@ -1458,7 +1458,8 @@ bool wasm::EnsureBuiltinThunksInitialized() {
   MOZ_ASSERT(masm.callSiteTargets().empty());
   MOZ_ASSERT(masm.trapSites().empty());
 
-  if (!ExecutableAllocator::makeExecutableAndFlushICache(thunks->codeBase,
+  if (!ExecutableAllocator::makeExecutableAndFlushICache(
+          FlushICacheSpec::LocalThreadOnly, thunks->codeBase,
           thunks->codeSize)) {
     return false;
   }

js/src/wasm/WasmCode.cpp

@@ -376,7 +376,7 @@ UniqueModuleSegment ModuleSegment::create(Tier tier, const Bytes& unlinkedBytes,
                                        linkData);
 }
 
-bool ModuleSegment::initialize(const CodeTier& codeTier,
+bool ModuleSegment::initialize(IsTier2 isTier2, const CodeTier& codeTier,
                                const LinkData& linkData,
                                const Metadata& metadata,
                                const MetadataTier& metadataTier) {
@@ -384,9 +384,15 @@ bool ModuleSegment::initialize(const CodeTier& codeTier,
     return false;
   }
 
+  // Optimized compilation finishes on a background thread, so we must make sure
+  // to flush the icaches of all the executing threads.
+  FlushICacheSpec flushIcacheSpec = isTier2 == IsTier2::Tier2
+                                        ? FlushICacheSpec::AllThreads
+                                        : FlushICacheSpec::LocalThreadOnly;
+
   // Reprotect the whole region to avoid having separate RW and RX mappings.
   if (!ExecutableAllocator::makeExecutableAndFlushICache(
-          base(), RoundupCodeLength(length()))) {
+          flushIcacheSpec, base(), RoundupCodeLength(length()))) {
     return false;
   }
 
@@ -661,6 +667,7 @@ static constexpr unsigned LAZY_STUB_LIFO_DEFAULT_CHUNK_SIZE = 8 * 1024;
 
 bool LazyStubTier::createMany(const Uint32Vector& funcExportIndices,
                               const CodeTier& codeTier,
+                              bool flushAllThreadsIcaches,
                               size_t* stubSegmentIndex) {
   MOZ_ASSERT(funcExportIndices.length());
 
@@ -739,7 +746,13 @@ bool LazyStubTier::createMany(const Uint32Vector& funcExportIndices,
     Assembler::Bind(codePtr, label);
   }
 
-  if (!ExecutableAllocator::makeExecutableAndFlushICache(codePtr, codeLength)) {
+  // Optimized compilation finishes on a background thread, so we must make sure
+  // to flush the icaches of all the executing threads.
+  FlushICacheSpec flushIcacheSpec = flushAllThreadsIcaches
+                                        ? FlushICacheSpec::AllThreads
+                                        : FlushICacheSpec::LocalThreadOnly;
+  if (!ExecutableAllocator::makeExecutableAndFlushICache(flushIcacheSpec,
+                                                         codePtr, codeLength)) {
     return false;
   }
 
@@ -785,8 +798,13 @@ bool LazyStubTier::createOne(uint32_t funcExportIndex,
     return false;
   }
 
+  // This happens on the executing thread (called via GetInterpEntry), so no
+  // need to flush the icaches on all the threads.
+  bool flushAllThreadIcaches = false;
+
   size_t stubSegmentIndex;
-  if (!createMany(funcExportIndexes, codeTier, &stubSegmentIndex)) {
+  if (!createMany(funcExportIndexes, codeTier, flushAllThreadIcaches,
+                  &stubSegmentIndex)) {
     return false;
   }
 
@@ -828,8 +846,13 @@ bool LazyStubTier::createTier2(const Uint32Vector& funcExportIndices,
     return true;
   }
 
+  // This compilation happens on a background compiler thread, so the icache may
+  // need to be flushed on all the threads.
+  bool flushAllThreadIcaches = true;
+
   size_t stubSegmentIndex;
-  if (!createMany(funcExportIndices, codeTier, &stubSegmentIndex)) {
+  if (!createMany(funcExportIndices, codeTier, flushAllThreadIcaches,
+                  &stubSegmentIndex)) {
     return false;
   }
 
@@ -1031,15 +1054,15 @@ bool Metadata::getFuncName(NameContext ctx, uint32_t funcIndex,
   return AppendFunctionIndexName(funcIndex, name);
 }
 
-bool CodeTier::initialize(const Code& code, const LinkData& linkData,
-                          const Metadata& metadata) {
+bool CodeTier::initialize(IsTier2 isTier2, const Code& code,
+                          const LinkData& linkData, const Metadata& metadata) {
   MOZ_ASSERT(!initialized());
   code_ = &code;
 
   MOZ_ASSERT(lazyStubs_.lock()->empty());
 
   // See comments in CodeSegment::initialize() for why this must be last.
-  if (!segment_->initialize(*this, linkData, metadata, *metadata_)) {
+  if (!segment_->initialize(isTier2, *this, linkData, metadata, *metadata_)) {
     return false;
   }
 
@@ -1150,7 +1173,7 @@ Code::Code(UniqueCodeTier tier1, const Metadata& metadata,
 bool Code::initialize(const LinkData& linkData) {
   MOZ_ASSERT(!initialized());
 
-  if (!tier1_->initialize(*this, linkData, *metadata_)) {
+  if (!tier1_->initialize(IsTier2::NotTier2, *this, linkData, *metadata_)) {
     return false;
   }
 
@@ -1163,7 +1186,7 @@ bool Code::setTier2(UniqueCodeTier tier2, const LinkData& linkData) const {
   MOZ_RELEASE_ASSERT(tier2->tier() == Tier::Optimized &&
                      tier1_->tier() == Tier::Baseline);
 
-  if (!tier2->initialize(*this, linkData, *metadata_)) {
+  if (!tier2->initialize(IsTier2::Tier2, *this, linkData, *metadata_)) {
     return false;
   }
 

js/src/wasm/WasmCode.h

@@ -162,6 +162,8 @@ class CodeSegment {
 
 using UniqueModuleSegment = UniquePtr<ModuleSegment>;
 
+enum IsTier2 { Tier2, NotTier2 };
+
 class ModuleSegment : public CodeSegment {
   const Tier tier_;
   uint8_t* const trapCode_;
@@ -175,8 +177,9 @@ class ModuleSegment : public CodeSegment {
   static UniqueModuleSegment create(Tier tier, const Bytes& unlinkedBytes,
                                     const LinkData& linkData);
 
-  bool initialize(const CodeTier& codeTier, const LinkData& linkData,
-                  const Metadata& metadata, const MetadataTier& metadataTier);
+  bool initialize(IsTier2 compileMode, const CodeTier& codeTier,
+                  const LinkData& linkData, const Metadata& metadata,
+                  const MetadataTier& metadataTier);
 
   Tier tier() const { return tier_; }
 
@@ -519,7 +522,8 @@ class LazyStubTier {
   size_t lastStubSegmentIndex_;
 
   bool createMany(const Uint32Vector& funcExportIndices,
-                  const CodeTier& codeTier, size_t* stubSegmentIndex);
+                  const CodeTier& codeTier, bool flushAllThreadsIcaches,
+                  size_t* stubSegmentIndex);
 
  public:
   LazyStubTier() : lastStubSegmentIndex_(0) {}
@@ -579,7 +583,7 @@ class CodeTier {
         lazyStubs_(mutexForTier(segment_->tier())) {}
 
   bool initialized() const { return !!code_ && segment_->initialized(); }
-  bool initialize(const Code& code, const LinkData& linkData,
+  bool initialize(IsTier2 isTier2, const Code& code, const LinkData& linkData,
                   const Metadata& metadata);
 
   Tier tier() const { return segment_->tier(); }

js/src/wasm/WasmCompile.cpp

@@ -449,6 +449,20 @@ void CompilerEnvironment::computeParameters() {
   state_ = Computed;
 }
 
+// Check that this architecture either:
+// - is cache-coherent, which is the case for most tier-1 architectures we care
+// about.
+// - or has the ability to invalidate the instruction cache of all threads, so
+// background compilation in tiered compilation can be synchronized across all
+// threads.
+static bool IsICacheSafe() {
+#ifdef JS_CODEGEN_ARM64
+  return jit::CanFlushICacheFromBackgroundThreads();
+#else
+  return true;
+#endif
+}
+
 void CompilerEnvironment::computeParameters(Decoder& d) {
   MOZ_ASSERT(!isComputed());
 
@@ -484,7 +498,7 @@ void CompilerEnvironment::computeParameters(Decoder& d) {
   }
 
   if (baselineEnabled && hasSecondTier && CanUseExtraThreads() &&
-      (TieringBeneficial(codeSectionSize) || forceTiering)) {
+      (TieringBeneficial(codeSectionSize) || forceTiering) && IsICacheSafe()) {
     mode_ = CompileMode::Tier1;
     tier_ = Tier::Baseline;
   } else {