// Copyright 2013 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef V8_V8_PLATFORM_H_
#define V8_V8_PLATFORM_H_
#include <stddef.h>
#include <stdint.h>
#include <stdlib.h> // For abort.
#include <memory>
#include <string>
#include "v8config.h" // NOLINT(build/include_directory)
namespace v8 {
class Isolate;
// Valid priorities supported by the task scheduling infrastructure.
enum class TaskPriority : uint8_t {
/**
* Best effort tasks are not critical for performance of the application. The
* platform implementation should preempt such tasks if higher priority tasks
* arrive.
*/
kBestEffort,
/**
* User visible tasks are long running background tasks that will
* improve performance and memory usage of the application upon completion.
* Example: background compilation and garbage collection.
*/
kUserVisible,
/**
* User blocking tasks are highest priority tasks that block the execution
* thread (e.g. major garbage collection). They must be finished as soon as
* possible.
*/
kUserBlocking,
};
/**
* A Task represents a unit of work.
*/
class Task {
public:
virtual ~Task() = default;
virtual void Run() = 0;
};
/**
* An IdleTask represents a unit of work to be performed in idle time.
* The Run method is invoked with an argument that specifies the deadline in
* seconds returned by MonotonicallyIncreasingTime().
* The idle task is expected to complete by this deadline.
*/
class IdleTask {
public:
virtual ~IdleTask() = default;
virtual void Run(double deadline_in_seconds) = 0;
};
/**
* A TaskRunner allows scheduling of tasks. The TaskRunner may still be used to
* post tasks after the isolate gets destructed, but these tasks may not get
* executed anymore. All tasks posted to a given TaskRunner will be invoked in
* sequence. Tasks can be posted from any thread.
*/
class TaskRunner {
public:
/**
* Schedules a task to be invoked by this TaskRunner. The TaskRunner
* implementation takes ownership of |task|.
*/
virtual void PostTask(std::unique_ptr<Task> task) = 0;
/**
* Schedules a task to be invoked by this TaskRunner. The TaskRunner
* implementation takes ownership of |task|. The |task| cannot be nested
* within other task executions.
*
* Tasks which shouldn't be interleaved with JS execution must be posted with
* |PostNonNestableTask| or |PostNonNestableDelayedTask|. This is because the
* embedder may process tasks in a callback which is called during JS
* execution.
*
* In particular, tasks which execute JS must be non-nestable, since JS
* execution is not allowed to nest.
*
* Requires that |TaskRunner::NonNestableTasksEnabled()| is true.
*/
virtual void PostNonNestableTask(std::unique_ptr<Task> task) {}
/**
* Schedules a task to be invoked by this TaskRunner. The task is scheduled
* after the given number of seconds |delay_in_seconds|. The TaskRunner
* implementation takes ownership of |task|.
*/
virtual void PostDelayedTask(std::unique_ptr<Task> task,
double delay_in_seconds) = 0;
/**
* Schedules a task to be invoked by this TaskRunner. The task is scheduled
* after the given number of seconds |delay_in_seconds|. The TaskRunner
* implementation takes ownership of |task|. The |task| cannot be nested
* within other task executions.
*
* Tasks which shouldn't be interleaved with JS execution must be posted with
* |PostNonNestableTask| or |PostNonNestableDelayedTask|. This is because the
* embedder may process tasks in a callback which is called during JS
* execution.
*
* In particular, tasks which execute JS must be non-nestable, since JS
* execution is not allowed to nest.
*
* Requires that |TaskRunner::NonNestableDelayedTasksEnabled()| is true.
*/
virtual void PostNonNestableDelayedTask(std::unique_ptr<Task> task,
double delay_in_seconds) {}
/**
* Schedules an idle task to be invoked by this TaskRunner. The task is
* scheduled when the embedder is idle. Requires that
* |TaskRunner::IdleTasksEnabled()| is true. Idle tasks may be reordered
* relative to other task types and may be starved for an arbitrarily long
* time if no idle time is available. The TaskRunner implementation takes
* ownership of |task|.
*/
virtual void PostIdleTask(std::unique_ptr<IdleTask> task) = 0;
/**
* Returns true if idle tasks are enabled for this TaskRunner.
*/
virtual bool IdleTasksEnabled() = 0;
/**
* Returns true if non-nestable tasks are enabled for this TaskRunner.
*/
virtual bool NonNestableTasksEnabled() const { return false; }
/**
* Returns true if non-nestable delayed tasks are enabled for this TaskRunner.
*/
virtual bool NonNestableDelayedTasksEnabled() const { return false; }
TaskRunner() = default;
virtual ~TaskRunner() = default;
TaskRunner(const TaskRunner&) = delete;
TaskRunner& operator=(const TaskRunner&) = delete;
};
/**
* Delegate that's passed to Job's worker task, providing an entry point to
* communicate with the scheduler.
*/
class JobDelegate {
public:
/**
* Returns true if this thread should return from the worker task on the
* current thread ASAP. Workers should periodically invoke ShouldYield (or
* YieldIfNeeded()) as often as is reasonable.
*/
virtual bool ShouldYield() = 0;
/**
* Notifies the scheduler that max concurrency was increased, and the number
* of worker should be adjusted accordingly. See Platform::PostJob() for more
* details.
*/
virtual void NotifyConcurrencyIncrease() = 0;
/**
* Returns a task_id unique among threads currently running this job, such
* that GetTaskId() < worker count. To achieve this, the same task_id may be
* reused by a different thread after a worker_task returns.
*/
virtual uint8_t GetTaskId() = 0;
/**
* Returns true if the current task is called from the thread currently
* running JobHandle::Join().
* TODO(etiennep): Make pure virtual once custom embedders implement it.
*/
virtual bool IsJoiningThread() const { return false; }
};
/**
* Handle returned when posting a Job. Provides methods to control execution of
* the posted Job.
*/
class JobHandle {
public:
virtual ~JobHandle() = default;
/**
* Notifies the scheduler that max concurrency was increased, and the number
* of worker should be adjusted accordingly. See Platform::PostJob() for more
* details.
*/
virtual void NotifyConcurrencyIncrease() = 0;
/**
* Contributes to the job on this thread. Doesn't return until all tasks have
* completed and max concurrency becomes 0. When Join() is called and max
* concurrency reaches 0, it should not increase again. This also promotes
* this Job's priority to be at least as high as the calling thread's
* priority.
*/
virtual void Join() = 0;
/**
* Forces all existing workers to yield ASAP. Waits until they have all
* returned from the Job's callback before returning.
*/
virtual void Cancel() = 0;
/*
* Forces all existing workers to yield ASAP but doesn’t wait for them.
* Warning, this is dangerous if the Job's callback is bound to or has access
* to state which may be deleted after this call.
* TODO(etiennep): Cleanup once implemented by all embedders.
*/
virtual void CancelAndDetach() { Cancel(); }
/**
* Returns true if there's any work pending or any worker running.
*/
virtual bool IsActive() = 0;
// TODO(etiennep): Clean up once all overrides are removed.
V8_DEPRECATED("Use !IsActive() instead.")
virtual bool IsCompleted() { return !IsActive(); }
/**
* Returns true if associated with a Job and other methods may be called.
* Returns false after Join() or Cancel() was called. This may return true
* even if no workers are running and IsCompleted() returns true
*/
virtual bool IsValid() = 0;
// TODO(etiennep): Clean up once all overrides are removed.
V8_DEPRECATED("Use IsValid() instead.")
virtual bool IsRunning() { return IsValid(); }
/**
* Returns true if job priority can be changed.
*/
virtual bool UpdatePriorityEnabled() const { return false; }
/**
* Update this Job's priority.
*/
virtual void UpdatePriority(TaskPriority new_priority) {}
};
/**
* A JobTask represents work to run in parallel from Platform::PostJob().
*/
class JobTask {
public:
virtual ~JobTask() = default;
virtual void Run(JobDelegate* delegate) = 0;
/**
* Controls the maximum number of threads calling Run() concurrently, given
* the number of threads currently assigned to this job and executing Run().
* Run() is only invoked if the number of threads previously running Run() was
* less than the value returned. Since GetMaxConcurrency() is a leaf function,
* it must not call back any JobHandle methods.
*/
virtual size_t GetMaxConcurrency(size_t worker_count) const = 0;
// TODO(1114823): Clean up once all overrides are removed.
V8_DEPRECATED("Use the version that takes |worker_count|.")
virtual size_t GetMaxConcurrency() const { return 0; }
};
/**
* The interface represents complex arguments to trace events.
*/
class ConvertableToTraceFormat {
public:
virtual ~ConvertableToTraceFormat() = default;
/**
* Append the class info to the provided |out| string. The appended
* data must be a valid JSON object. Strings must be properly quoted, and
* escaped. There is no processing applied to the content after it is
* appended.
*/
virtual void AppendAsTraceFormat(std::string* out) const = 0;
};
/**
* V8 Tracing controller.
*
* Can be implemented by an embedder to record trace events from V8.
*/
class TracingController {
public:
virtual ~TracingController() = default;
// In Perfetto mode, trace events are written using Perfetto's Track Event
// API directly without going through the embedder. However, it is still
// possible to observe tracing being enabled and disabled.
#if !defined(V8_USE_PERFETTO)
/**
* Called by TRACE_EVENT* macros, don't call this directly.
* The name parameter is a category group for example:
* TRACE_EVENT0("v8,parse", "V8.Parse")
* The pointer returned points to a value with zero or more of the bits
* defined in CategoryGroupEnabledFlags.
**/
virtual const uint8_t* GetCategoryGroupEnabled(const char* name) {
static uint8_t no = 0;
return &no;
}
/**
* Adds a trace event to the platform tracing system. These function calls are
* usually the result of a TRACE_* macro from trace_event_common.h when
* tracing and the category of the particular trace are enabled. It is not
* advisable to call these functions on their own; they are really only meant
* to be used by the trace macros. The returned handle can be used by
* UpdateTraceEventDuration to update the duration of COMPLETE events.
*/
virtual uint64_t AddTraceEvent(
char phase, const uint8_t* category_enabled_flag, const char* name,
const char* scope, uint64_t id, uint64_t bind_id, int32_t num_args,
const char** arg_names, const uint8_t* arg_types,
const uint64_t* arg_values,
std::unique_ptr<ConvertableToTraceFormat>* arg_convertables,
unsigned int flags) {
return 0;
}
virtual uint64_t AddTraceEventWithTimestamp(
char phase, const uint8_t* category_enabled_flag, const char* name,
const char* scope, uint64_t id, uint64_t bind_id, int32_t num_args,
const char** arg_names, const uint8_t* arg_types,
const uint64_t* arg_values,
std::unique_ptr<ConvertableToTraceFormat>* arg_convertables,
unsigned int flags, int64_t timestamp) {
return 0;
}
/**
* Sets the duration field of a COMPLETE trace event. It must be called with
* the handle returned from AddTraceEvent().
**/
virtual void UpdateTraceEventDuration(const uint8_t* category_enabled_flag,
const char* name, uint64_t handle) {}
#endif // !defined(V8_USE_PERFETTO)
class TraceStateObserver {
public:
virtual ~TraceStateObserver() = default;
virtual void OnTraceEnabled() = 0;
virtual void OnTraceDisabled() = 0;
};
/** Adds tracing state change observer. */
virtual void AddTraceStateObserver(TraceStateObserver*) {}
/** Removes tracing state change observer. */
virtual void RemoveTraceStateObserver(TraceStateObserver*) {}
};
/**
* A V8 memory page allocator.
*
* Can be implemented by an embedder to manage large host OS allocations.
*/
class PageAllocator {
public:
virtual ~PageAllocator() = default;
/**
* Gets the page granularity for AllocatePages and FreePages. Addresses and
* lengths for those calls should be multiples of AllocatePageSize().
*/
virtual size_t AllocatePageSize() = 0;
/**
* Gets the page granularity for SetPermissions and ReleasePages. Addresses
* and lengths for those calls should be multiples of CommitPageSize().
*/
virtual size_t CommitPageSize() = 0;
/**
* Sets the random seed so that GetRandomMmapAddr() will generate repeatable
* sequences of random mmap addresses.
*/
virtual void SetRandomMmapSeed(int64_t seed) = 0;
/**
* Returns a randomized address, suitable for memory allocation under ASLR.
* The address will be aligned to AllocatePageSize.
*/
virtual void* GetRandomMmapAddr() = 0;
/**
* Memory permissions.
*/
enum Permission {
kNoAccess,
kRead,
kReadWrite,
kReadWriteExecute,
kReadExecute,
// Set this when reserving memory that will later require kReadWriteExecute
// permissions. The resulting behavior is platform-specific, currently
// this is used to set the MAP_JIT flag on Apple Silicon.
// TODO(jkummerow): Remove this when Wasm has a platform-independent
// w^x implementation.
kNoAccessWillJitLater
};
/**
* Allocates memory in range with the given alignment and permission.
*/
virtual void* AllocatePages(void* address, size_t length, size_t alignment,
Permission permissions) = 0;
/**
* Frees memory in a range that was allocated by a call to AllocatePages.
*/
virtual bool FreePages(void* address, size_t length) = 0;
/**
* Releases memory in a range that was allocated by a call to AllocatePages.
*/
virtual bool ReleasePages(void* address, size_t length,
size_t new_length) = 0;
/**
* Sets permissions on pages in an allocated range.
*/
virtual bool SetPermissions(void* address, size_t length,
Permission permissions) = 0;
/**
* Frees memory in the given [address, address + size) range. address and size
* should be operating system page-aligned. The next write to this
* memory area brings the memory transparently back.
*/
virtual bool DiscardSystemPages(void* address, size_t size) { return true; }
/**
* INTERNAL ONLY: This interface has not been stabilised and may change
* without notice from one release to another without being deprecated first.
*/
class SharedMemoryMapping {
public:
// Implementations are expected to free the shared memory mapping in the
// destructor.
virtual ~SharedMemoryMapping() = default;
virtual void* GetMemory() const = 0;
};
/**
* INTERNAL ONLY: This interface has not been stabilised and may change
* without notice from one release to another without being deprecated first.
*/
class SharedMemory {
public:
// Implementations are expected to free the shared memory in the destructor.
virtual ~SharedMemory() = default;
virtual std::unique_ptr<SharedMemoryMapping> RemapTo(
void* new_address) const = 0;
virtual void* GetMemory() const = 0;
virtual size_t GetSize() const = 0;
};
/**
* INTERNAL ONLY: This interface has not been stabilised and may change
* without notice from one release to another without being deprecated first.
*
* Reserve pages at a fixed address returning whether the reservation is
* possible. The reserved memory is detached from the PageAllocator and so
* should not be freed by it. It's intended for use with
* SharedMemory::RemapTo, where ~SharedMemoryMapping would free the memory.
*/
virtual bool ReserveForSharedMemoryMapping(void* address, size_t size) {
return false;
}
/**
* INTERNAL ONLY: This interface has not been stabilised and may change
* without notice from one release to another without being deprecated first.
*
* Allocates shared memory pages. Not all PageAllocators need support this and
* so this method need not be overridden.
* Allocates a new read-only shared memory region of size |length| and copies
* the memory at |original_address| into it.
*/
virtual std::unique_ptr<SharedMemory> AllocateSharedPages(
size_t length, const void* original_address) {
return {};
}
/**
* INTERNAL ONLY: This interface has not been stabilised and may change
* without notice from one release to another without being deprecated first.
*
* If not overridden and changed to return true, V8 will not attempt to call
* AllocateSharedPages or RemapSharedPages. If overridden, AllocateSharedPages
* and RemapSharedPages must also be overridden.
*/
virtual bool CanAllocateSharedPages() { return false; }
};
/**
* V8 Platform abstraction layer.
*
* The embedder has to provide an implementation of this interface before
* initializing the rest of V8.
*/
class Platform {
public:
virtual ~Platform() = default;
/**
* Allows the embedder to manage memory page allocations.
*/
virtual PageAllocator* GetPageAllocator() {
// TODO(bbudge) Make this abstract after all embedders implement this.
return nullptr;
}
/**
* Enables the embedder to respond in cases where V8 can't allocate large
* blocks of memory. V8 retries the failed allocation once after calling this
* method. On success, execution continues; otherwise V8 exits with a fatal
* error.
* Embedder overrides of this function must NOT call back into V8.
*/
virtual void OnCriticalMemoryPressure() {
// TODO(bbudge) Remove this when embedders override the following method.
// See crbug.com/634547.
}
/**
* Enables the embedder to respond in cases where V8 can't allocate large
* memory regions. The |length| parameter is the amount of memory needed.
* Returns true if memory is now available. Returns false if no memory could
* be made available. V8 will retry allocations until this method returns
* false.
*
* Embedder overrides of this function must NOT call back into V8.
*/
virtual bool OnCriticalMemoryPressure(size_t length) { return false; }
/**
* Gets the number of worker threads used by
* Call(BlockingTask)OnWorkerThread(). This can be used to estimate the number
* of tasks a work package should be split into. A return value of 0 means
* that there are no worker threads available. Note that a value of 0 won't
* prohibit V8 from posting tasks using |CallOnWorkerThread|.
*/
virtual int NumberOfWorkerThreads() = 0;
/**
* Returns a TaskRunner which can be used to post a task on the foreground.
* The TaskRunner's NonNestableTasksEnabled() must be true. This function
* should only be called from a foreground thread.
*/
virtual std::shared_ptr<v8::TaskRunner> GetForegroundTaskRunner(
Isolate* isolate) = 0;
/**
* Schedules a task to be invoked on a worker thread.
*/
virtual void CallOnWorkerThread(std::unique_ptr<Task> task) = 0;
/**
* Schedules a task that blocks the main thread to be invoked with
* high-priority on a worker thread.
*/
virtual void CallBlockingTaskOnWorkerThread(std::unique_ptr<Task> task) {
// Embedders may optionally override this to process these tasks in a high
// priority pool.
CallOnWorkerThread(std::move(task));
}
/**
* Schedules a task to be invoked with low-priority on a worker thread.
*/
virtual void CallLowPriorityTaskOnWorkerThread(std::unique_ptr<Task> task) {
// Embedders may optionally override this to process these tasks in a low
// priority pool.
CallOnWorkerThread(std::move(task));
}
/**
* Schedules a task to be invoked on a worker thread after |delay_in_seconds|
* expires.
*/
virtual void CallDelayedOnWorkerThread(std::unique_ptr<Task> task,
double delay_in_seconds) = 0;
/**
* Returns true if idle tasks are enabled for the given |isolate|.
*/
virtual bool IdleTasksEnabled(Isolate* isolate) { return false; }
/**
* Posts |job_task| to run in parallel. Returns a JobHandle associated with
* the Job, which can be joined or canceled.
* This avoids degenerate cases:
* - Calling CallOnWorkerThread() for each work item, causing significant
* overhead.
* - Fixed number of CallOnWorkerThread() calls that split the work and might
* run for a long time. This is problematic when many components post
* "num cores" tasks and all expect to use all the cores. In these cases,
* the scheduler lacks context to be fair to multiple same-priority requests
* and/or ability to request lower priority work to yield when high priority
* work comes in.
* A canonical implementation of |job_task| looks like:
* class MyJobTask : public JobTask {
* public:
* MyJobTask(...) : worker_queue_(...) {}
* // JobTask:
* void Run(JobDelegate* delegate) override {
* while (!delegate->ShouldYield()) {
* // Smallest unit of work.
* auto work_item = worker_queue_.TakeWorkItem(); // Thread safe.
* if (!work_item) return;
* ProcessWork(work_item);
* }
* }
*
* size_t GetMaxConcurrency() const override {
* return worker_queue_.GetSize(); // Thread safe.
* }
* };
* auto handle = PostJob(TaskPriority::kUserVisible,
* std::make_unique<MyJobTask>(...));
* handle->Join();
*
* PostJob() and methods of the returned JobHandle/JobDelegate, must never be
* called while holding a lock that could be acquired by JobTask::Run or
* JobTask::GetMaxConcurrency -- that could result in a deadlock. This is
* because [1] JobTask::GetMaxConcurrency may be invoked while holding
* internal lock (A), hence JobTask::GetMaxConcurrency can only use a lock (B)
* if that lock is *never* held while calling back into JobHandle from any
* thread (A=>B/B=>A deadlock) and [2] JobTask::Run or
* JobTask::GetMaxConcurrency may be invoked synchronously from JobHandle
* (B=>JobHandle::foo=>B deadlock).
*
* A sufficient PostJob() implementation that uses the default Job provided in
* libplatform looks like:
* std::unique_ptr<JobHandle> PostJob(
* TaskPriority priority, std::unique_ptr<JobTask> job_task) override {
* return v8::platform::NewDefaultJobHandle(
* this, priority, std::move(job_task), NumberOfWorkerThreads());
* }
*/
virtual std::unique_ptr<JobHandle> PostJob(
TaskPriority priority, std::unique_ptr<JobTask> job_task) = 0;
/**
* Monotonically increasing time in seconds from an arbitrary fixed point in
* the past. This function is expected to return at least
* millisecond-precision values. For this reason,
* it is recommended that the fixed point be no further in the past than
* the epoch.
**/
virtual double MonotonicallyIncreasingTime() = 0;
/**
* Current wall-clock time in milliseconds since epoch.
* This function is expected to return at least millisecond-precision values.
*/
virtual double CurrentClockTimeMillis() = 0;
typedef void (*StackTracePrinter)();
/**
* Returns a function pointer that print a stack trace of the current stack
* on invocation. Disables printing of the stack trace if nullptr.
*/
virtual StackTracePrinter GetStackTracePrinter() { return nullptr; }
/**
* Returns an instance of a v8::TracingController. This must be non-nullptr.
*/
virtual TracingController* GetTracingController() = 0;
/**
* Tells the embedder to generate and upload a crashdump during an unexpected
* but non-critical scenario.
*/
virtual void DumpWithoutCrashing() {}
protected:
/**
* Default implementation of current wall-clock time in milliseconds
* since epoch. Useful for implementing |CurrentClockTimeMillis| if
* nothing special needed.
*/
V8_EXPORT static double SystemClockTimeMillis();
};
} // namespace v8
#endif // V8_V8_PLATFORM_H_
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