Index: lldb/include/lldb/Utility/Diagnostics.h
===================================================================
--- lldb/include/lldb/Utility/Diagnostics.h
+++ lldb/include/lldb/Utility/Diagnostics.h
@@ -11,6 +11,7 @@
 
 #include "lldb/Utility/FileSpec.h"
 #include "lldb/Utility/Log.h"
+#include "llvm/ADT/BitmaskEnum.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringSet.h"
@@ -22,6 +23,21 @@
 
 namespace lldb_private {
 
+enum class DiagnosticsLog : Log::MaskType {
+  LLDB = Log::ChannelFlag<0>,
+  LLVM_MARK_AS_BITMASK_ENUM(LLDB)
+};
+
+LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE();
+
+class LogChannelDiagnostics {
+public:
+  static void Initialize();
+  static void Terminate();
+};
+
+template <> Log::Channel &LogChannelFor<DiagnosticsLog>();
+
 /// Diagnostics are a collection of files to help investigate bugs and
 /// troubleshoot issues. Any part of the debugger can register itself with the
 /// help of a callback to emit one or more files into the diagnostic directory.
@@ -47,6 +63,10 @@
 private:
   static llvm::Optional<Diagnostics> &InstanceImpl();
 
+  llvm::Error DumpAlwaysOnLog(const FileSpec &dir) const;
+
+  std::shared_ptr<RotatingLogHandler> m_always_on_log_handler;
+
   llvm::SmallVector<Callback, 4> m_callbacks;
   std::mutex m_callbacks_mutex;
 };
Index: lldb/include/lldb/Utility/Log.h
===================================================================
--- lldb/include/lldb/Utility/Log.h
+++ lldb/include/lldb/Utility/Log.h
@@ -150,12 +150,13 @@
   public:
     const llvm::ArrayRef<Category> categories;
     const MaskType default_flags;
+    const bool internal = false;
 
     template <typename Cat>
     constexpr Channel(llvm::ArrayRef<Log::Category> categories,
-                      Cat default_flags)
+                      Cat default_flags, bool internal = false)
         : log_ptr(nullptr), categories(categories),
-          default_flags(MaskType(default_flags)) {
+          default_flags(MaskType(default_flags)), internal(internal) {
       static_assert(
           std::is_same<Log::MaskType, std::underlying_type_t<Cat>>::value);
     }
@@ -341,6 +342,14 @@
       log_private->Format(__FILE__, __func__, __VA_ARGS__);                    \
   } while (0)
 
+#define LLDB_MULTI_LOG(log1, log2, ...)                                        \
+  do {                                                                         \
+    if (::lldb_private::Log *log_private = (log1))                             \
+      log_private->Format(__FILE__, __func__, __VA_ARGS__);                    \
+    if (::lldb_private::Log *log_private = (log2))                             \
+      log_private->Format(__FILE__, __func__, __VA_ARGS__);                    \
+  } while (0)
+
 #define LLDB_LOGF(log, ...)                                                    \
   do {                                                                         \
     ::lldb_private::Log *log_private = (log);                                  \
@@ -348,6 +357,14 @@
       log_private->Printf(__VA_ARGS__);                                        \
   } while (0)
 
+#define LLDB_MULTI_LOGF(log1, log2, ...)                                       \
+  do {                                                                         \
+    if (::lldb_private::Log *log_private = (log1))                             \
+      log_private->Printf(__VA_ARGS__);                                        \
+    if (::lldb_private::Log *log_private = (log2))                             \
+      log_private->Printf(__VA_ARGS__);                                        \
+  } while (0)
+
 #define LLDB_LOGV(log, ...)                                                    \
   do {                                                                         \
     ::lldb_private::Log *log_private = (log);                                  \
Index: lldb/source/Utility/Diagnostics.cpp
===================================================================
--- lldb/source/Utility/Diagnostics.cpp
+++ lldb/source/Utility/Diagnostics.cpp
@@ -17,7 +17,27 @@
 using namespace lldb;
 using namespace llvm;
 
+static constexpr size_t g_num_log_messages = 100;
+
+static constexpr Log::Category g_categories[] = {
+    {{"lldb"}, {"diagnostics log for lldb"}, DiagnosticsLog::LLDB},
+};
+
+static Log::Channel g_channel(g_categories, DiagnosticsLog::LLDB,
+                              /*internal=*/true);
+
+template <> Log::Channel &lldb_private::LogChannelFor<DiagnosticsLog>() {
+  return g_channel;
+}
+
+void LogChannelDiagnostics::Initialize() {
+  Log::Register("diagnostics", g_channel);
+}
+
+void LogChannelDiagnostics::Terminate() { Log::Unregister("diagnostics"); }
+
 void Diagnostics::Initialize() {
+  LogChannelDiagnostics::Initialize();
   lldbassert(!InstanceImpl() && "Already initialized.");
   InstanceImpl().emplace();
 }
@@ -25,6 +45,7 @@
 void Diagnostics::Terminate() {
   lldbassert(InstanceImpl() && "Already terminated.");
   InstanceImpl().reset();
+  LogChannelDiagnostics::Terminate();
 }
 
 Optional<Diagnostics> &Diagnostics::InstanceImpl() {
@@ -34,9 +55,18 @@
 
 Diagnostics &Diagnostics::Instance() { return *InstanceImpl(); }
 
-Diagnostics::Diagnostics() {}
+Diagnostics::Diagnostics()
+    : m_always_on_log_handler(
+          std::make_shared<RotatingLogHandler>(g_num_log_messages)) {
+  const uint32_t log_options = LLDB_LOG_OPTION_PREPEND_THREAD_NAME;
+  Log::EnableLogChannel(m_always_on_log_handler, log_options, "diagnostics",
+                        {"all"}, llvm::nulls());
+  AddCallback([&](const FileSpec &dir) { return DumpAlwaysOnLog(dir); });
+}
 
-Diagnostics::~Diagnostics() {}
+Diagnostics::~Diagnostics() {
+  Log::DisableLogChannel("diagnostics", {"all"}, llvm::nulls());
+}
 
 void Diagnostics::AddCallback(Callback callback) {
   std::lock_guard<std::mutex> guard(m_callbacks_mutex);
@@ -66,9 +96,22 @@
 }
 
 Error Diagnostics::Create(const FileSpec &dir) {
+  LLDB_LOG(GetLog(DiagnosticsLog::LLDB), "Dumping {0} diagnostics to '{1}'",
+           m_callbacks.size(), dir.GetPath());
+
   for (Callback c : m_callbacks) {
     if (Error err = c(dir))
       return err;
   }
   return Error::success();
 }
+
+llvm::Error Diagnostics::DumpAlwaysOnLog(const FileSpec &dir) const {
+  FileSpec log_file = dir.CopyByAppendingPathComponent("always-on.log");
+  std::error_code ec;
+  llvm::raw_fd_ostream stream(log_file.GetPath(), ec, llvm::sys::fs::OF_None);
+  if (ec)
+    return errorCodeToError(ec);
+  m_always_on_log_handler->Dump(stream);
+  return Error::success();
+}
Index: lldb/source/Utility/Log.cpp
===================================================================
--- lldb/source/Utility/Log.cpp
+++ lldb/source/Utility/Log.cpp
@@ -283,8 +283,10 @@
 
 std::vector<llvm::StringRef> Log::ListChannels() {
   std::vector<llvm::StringRef> result;
-  for (const auto &channel : *g_channel_map)
-    result.push_back(channel.first());
+  for (const auto &entry : *g_channel_map) {
+    if (!entry.second.m_channel.internal)
+      result.push_back(entry.first());
+  }
   return result;
 }
 
@@ -294,8 +296,10 @@
     return;
   }
 
-  for (const auto &channel : *g_channel_map)
-    ListCategories(stream, channel);
+  for (const auto &entry : *g_channel_map) {
+    if (!entry.second.m_channel.internal)
+      ListCategories(stream, entry);
+  }
 }
 
 bool Log::GetVerbose() const {
Index: lldb/test/Shell/Diagnostics/TestLogChannel.test
===================================================================
--- /dev/null
+++ lldb/test/Shell/Diagnostics/TestLogChannel.test
@@ -0,0 +1,2 @@
+# RUN: %lldb -o 'log list' 2>&1 | FileCheck %s
+# CHECK-NOT: Logging categories for 'diagnostics'