Struct wiggle::wasmtime_crate::Config[−][src]

pub struct Config { /* fields omitted */ }
Expand description

Global configuration options used to create an Engine and customize its behavior.

This structure exposed a builder-like interface and is primarily consumed by Engine::new()

Implementations

Creates a new configuration object with the default configuration specified.

Sets the target triple for the Config.

By default, the host target triple is used for the Config.

This method can be used to change the target triple.

Cranelift flags will not be inferred for the given target and any existing target-specific Cranelift flags will be cleared.

Errors

This method will error if the given target triple is not supported.

Whether or not to enable support for asynchronous functions in Wasmtime.

When enabled, the config can optionally define host functions with async. Instances created and functions called with this Config must be called through their asynchronous APIs, however. For example using Func::call will panic when used with this config.

Asynchronous Wasm

WebAssembly does not currently have a way to specify at the bytecode level what is and isn’t async. Host-defined functions, however, may be defined as async. WebAssembly imports always appear synchronous, which gives rise to a bit of an impedance mismatch here. To solve this Wasmtime supports “asynchronous configs” which enables calling these asynchronous functions in a way that looks synchronous to the executing WebAssembly code.

An asynchronous config must always invoke wasm code asynchronously, meaning we’ll always represent its computation as a Future. The poll method of the futures returned by Wasmtime will perform the actual work of calling the WebAssembly. Wasmtime won’t manage its own thread pools or similar, that’s left up to the embedder.

To implement futures in a way that WebAssembly sees asynchronous host functions as synchronous, all async Wasmtime futures will execute on a separately allocated native stack from the thread otherwise executing Wasmtime. This separate native stack can then be switched to and from. Using this whenever an async host function returns a future that resolves to Pending we switch away from the temporary stack back to the main stack and propagate the Pending status.

In general it’s encouraged that the integration with async and wasmtime is designed early on in your embedding of Wasmtime to ensure that it’s planned that WebAssembly executes in the right context of your application.

Execution in poll

The Future::poll method is the main driving force behind Rust’s futures. That method’s own documentation states “an implementation of poll should strive to return quickly, and should not block”. This, however, can be at odds with executing WebAssembly code as part of the poll method itself. If your WebAssembly is untrusted then this could allow the poll method to take arbitrarily long in the worst case, likely blocking all other asynchronous tasks.

To remedy this situation you have a two possible ways to solve this:

  • First you can spawn futures into a thread pool. By doing this in a thread pool you are relaxing the requirement that Future::poll must be fast because your future is executing on a separate thread. This strategy, however, would likely still require some form of cancellation via crate::Store::interrupt_handle to ensure wasm doesn’t take too long to execute.

  • Alternatively you can enable the Config::consume_fuel method as well as crate::Store::out_of_fuel_async_yield When doing so this will configure Wasmtime futures to yield periodically while they’re executing WebAssembly code. After consuming the specified amount of fuel wasm futures will return Poll::Pending from their poll method, and will get automatically re-polled later. This enables the Future::poll method to take roughly a fixed amount of time since fuel is guaranteed to get consumed while wasm is executing. Note that to prevent infinite execution of wasm you’ll need to use either crate::Store::interrupt_handle or a normal timeout on futures (which will get triggered due to periodic polls).

In either case special care needs to be taken when integrating asynchronous wasm into your application. You should carefully plan where WebAssembly will execute and what compute resources will be allotted to it. If Wasmtime doesn’t support exactly what you’d like just yet, please feel free to open an issue!

Configures whether DWARF debug information will be emitted during compilation.

By default this option is false.

Configures whether backtraces in Trap will parse debug info in the wasm file to have filename/line number information.

When enabled this will causes modules to retain debugging information found in wasm binaries. This debug information will be used when a trap happens to symbolicate each stack frame and attempt to print a filename/line number for each wasm frame in the stack trace.

By default this option is WasmBacktraceDetails::Environment, meaning that wasm will read WASMTIME_BACKTRACE_DETAILS to indicate whether details should be parsed.

Configures whether functions and loops will be interruptable via the Store::interrupt_handle method.

For more information see the documentation on Store::interrupt_handle.

By default this option is false.

Configures whether execution of WebAssembly will “consume fuel” to either halt or yield execution as desired.

This option is similar in purpose to Config::interruptable where you can prevent infinitely-executing WebAssembly code. The difference is that this option allows deterministic execution of WebAssembly code by instrumenting generated code consume fuel as it executes. When fuel runs out the behavior is defined by configuration within a Store, and by default a trap is raised.

Note that a Store starts with no fuel, so if you enable this option you’ll have to be sure to pour some fuel into Store before executing some code.

By default this option is false.

Configures the maximum amount of stack space available for executing WebAssembly code.

WebAssembly has well-defined semantics on stack overflow. This is intended to be a knob which can help configure how much stack space wasm execution is allowed to consume. Note that the number here is not super-precise, but rather wasm will take at most “pretty close to this much” stack space.

If a wasm call (or series of nested wasm calls) take more stack space than the size specified then a stack overflow trap will be raised.

When the async feature is enabled, this value cannot exceed the async_stack_size option. Be careful not to set this value too close to async_stack_size as doing so may limit how much stack space is available for host functions. Unlike wasm functions that trap on stack overflow, a host function that overflows the stack will abort the process.

By default this option is 1 MiB.

Configures the size of the stacks used for asynchronous execution.

This setting configures the size of the stacks that are allocated for asynchronous execution. The value cannot be less than max_wasm_stack.

The amount of stack space guaranteed for host functions is async_stack_size - max_wasm_stack, so take care not to set these two values close to one another; doing so may cause host functions to overflow the stack and abort the process.

By default this option is 2 MiB.

Configures whether the WebAssembly threads proposal will be enabled for compilation.

The WebAssembly threads proposal is not currently fully standardized and is undergoing development. Additionally the support in wasmtime itself is still being worked on. Support for this feature can be enabled through this method for appropriate wasm modules.

This feature gates items such as shared memories and atomic instructions. Note that enabling the threads feature will also enable the bulk memory feature.

This is false by default.

Note: Wasmtime does not implement everything for the wasm threads spec at this time, so bugs, panics, and possibly segfaults should be expected. This should not be enabled in a production setting right now.

Configures whether the WebAssembly reference types proposal will be enabled for compilation.

This feature gates items such as the externref and funcref types as well as allowing a module to define multiple tables.

Note that enabling the reference types feature will also enable the bulk memory feature.

This is true by default on x86-64, and false by default on other architectures.

Configures whether the WebAssembly SIMD proposal will be enabled for compilation.

The WebAssembly SIMD proposal is not currently fully standardized and is undergoing development. Additionally the support in wasmtime itself is still being worked on. Support for this feature can be enabled through this method for appropriate wasm modules.

This feature gates items such as the v128 type and all of its operators being in a module.

This is false by default.

Note: Wasmtime does not implement everything for the wasm simd spec at this time, so bugs, panics, and possibly segfaults should be expected. This should not be enabled in a production setting right now.

Configures whether the WebAssembly bulk memory operations proposal will be enabled for compilation.

This feature gates items such as the memory.copy instruction, passive data/table segments, etc, being in a module.

This is true by default.

Configures whether the WebAssembly multi-value proposal will be enabled for compilation.

This feature gates functions and blocks returning multiple values in a module, for example.

This is true by default.

Configures whether the WebAssembly multi-memory proposal will be enabled for compilation.

This feature gates modules having more than one linear memory declaration or import.

This is false by default.

Configures whether the WebAssembly module linking proposal will be enabled for compilation.

Note that development of this feature is still underway, so enabling this is likely to be full of bugs.

This is false by default.

Configures whether the WebAssembly memory64 proposal will be enabled for compilation.

Note that this the upstream specification is not finalized and Wasmtime may also have bugs for this feature since it hasn’t been exercised much.

This is false by default.

Configures which compilation strategy will be used for wasm modules.

This method can be used to configure which compiler is used for wasm modules, and for more documentation consult the Strategy enumeration and its documentation.

The default value for this is Strategy::Auto.

Errors

Some compilation strategies require compile-time options of wasmtime itself to be set, but if they’re not set and the strategy is specified here then an error will be returned.

Creates a default profiler based on the profiling strategy chosen.

Profiler creation calls the type’s default initializer where the purpose is really just to put in place the type used for profiling.

Configures whether the debug verifier of Cranelift is enabled or not.

When Cranelift is used as a code generation backend this will configure it to have the enable_verifier flag which will enable a number of debug checks inside of Cranelift. This is largely only useful for the developers of wasmtime itself.

The default value for this is false

Configures the Cranelift code generator optimization level.

When the Cranelift code generator is used you can configure the optimization level used for generated code in a few various ways. For more information see the documentation of OptLevel.

The default value for this is OptLevel::None.

Configures whether Cranelift should perform a NaN-canonicalization pass.

When Cranelift is used as a code generation backend this will configure it to replace NaNs with a single canonical value. This is useful for users requiring entirely deterministic WebAssembly computation. This is not required by the WebAssembly spec, so it is not enabled by default.

The default value for this is false

Allows setting a Cranelift boolean flag or preset. This allows fine-tuning of Cranelift settings.

Since Cranelift flags may be unstable, this method should not be considered to be stable either; other Config functions should be preferred for stability.

Safety

This is marked as unsafe, because setting the wrong flag might break invariants, resulting in execution hazards.

Errors

This method can fail if the flag’s name does not exist.

Allows settings another Cranelift flag defined by a flag name and value. This allows fine-tuning of Cranelift settings.

Since Cranelift flags may be unstable, this method should not be considered to be stable either; other Config functions should be preferred for stability.

Note that this is marked as unsafe, because setting the wrong flag might break invariants, resulting in execution hazards.

Errors

This method can fail if the flag’s name does not exist, or the value is not appropriate for the flag type.

Loads cache configuration specified at path.

This method will read the file specified by path on the filesystem and attempt to load cache configuration from it. This method can also fail due to I/O errors, misconfiguration, syntax errors, etc. For expected syntax in the configuration file see the documentation online.

By default cache configuration is not enabled or loaded.

This method is only available when the cache feature of this crate is enabled.

Errors

This method can fail due to any error that happens when loading the file pointed to by path and attempting to load the cache configuration.

Loads cache configuration from the system default path.

This commit is the same as Config::cache_config_load except that it does not take a path argument and instead loads the default configuration present on the system. This is located, for example, on Unix at $HOME/.config/wasmtime/config.toml and is typically created with the wasmtime config new command.

By default cache configuration is not enabled or loaded.

This method is only available when the cache feature of this crate is enabled.

Errors

This method can fail due to any error that happens when loading the default system configuration. Note that it is not an error if the default config file does not exist, in which case the default settings for an enabled cache are applied.

Sets a custom memory creator.

Custom memory creators are used when creating host Memory objects or when creating instance linear memories for the on-demand instance allocation strategy.

Sets the instance allocation strategy to use.

When using the pooling instance allocation strategy, all linear memories will be created as “static” and the Config::static_memory_maximum_size and Config::static_memory_guard_size options will be used to configure the virtual memory allocations of linear memories.

Sets whether or not an attempt is made to initialize linear memories by page.

This setting is false by default and Wasmtime initializes linear memories by copying individual data segments from the compiled module.

Setting this to true will cause compilation to attempt to organize the data segments into WebAssembly pages and linear memories are initialized by copying each page rather than individual data segments.

Modules that import a memory or have data segments that use a global base will continue to be initialized by copying each data segment individually.

When combined with the uffd feature on Linux, this will allow Wasmtime to delay initialization of a linear memory page until it is accessed for the first time during WebAssembly execution; this may improve instantiation performance as a result.

Configures the maximum size, in bytes, where a linear memory is considered static, above which it’ll be considered dynamic.

Note: this value has important performance ramifications, be sure to understand what this value does before tweaking it and benchmarking.

This function configures the threshold for wasm memories whether they’re implemented as a dynamically relocatable chunk of memory or a statically located chunk of memory. The max_size parameter here is the size, in bytes, where if the maximum size of a linear memory is below max_size then it will be statically allocated with enough space to never have to move. If the maximum size of a linear memory is larger than max_size then wasm memory will be dynamically located and may move in memory through growth operations.

Specifying a max_size of 0 means that all memories will be dynamic and may be relocated through memory.grow. Also note that if any wasm memory’s maximum size is below max_size then it will still reserve max_size bytes in the virtual memory space.

Static vs Dynamic Memory

Linear memories represent contiguous arrays of bytes, but they can also be grown through the API and wasm instructions. When memory is grown if space hasn’t been preallocated then growth may involve relocating the base pointer in memory. Memories in Wasmtime are classified in two different ways:

  • static - these memories preallocate all space necessary they’ll ever need, meaning that the base pointer of these memories is never moved. Static memories may take more virtual memory space because of pre-reserving space for memories.

  • dynamic - these memories are not preallocated and may move during growth operations. Dynamic memories consume less virtual memory space because they don’t need to preallocate space for future growth.

Static memories can be optimized better in JIT code because once the base address is loaded in a function it’s known that we never need to reload it because it never changes, memory.grow is generally a pretty fast operation because the wasm memory is never relocated, and under some conditions bounds checks can be elided on memory accesses.

Dynamic memories can’t be quite as heavily optimized because the base address may need to be reloaded more often, they may require relocating lots of data on memory.grow, and dynamic memories require unconditional bounds checks on all memory accesses.

Should you use static or dynamic memory?

In general you probably don’t need to change the value of this property. The defaults here are optimized for each target platform to consume a reasonable amount of physical memory while also generating speedy machine code.

One of the main reasons you may want to configure this today is if your environment can’t reserve virtual memory space for each wasm linear memory. On 64-bit platforms wasm memories require a 6GB reservation by default, and system limits may prevent this in some scenarios. In this case you may wish to force memories to be allocated dynamically meaning that the virtual memory footprint of creating a wasm memory should be exactly what’s used by the wasm itself.

For 32-bit memories a static memory must contain at least 4GB of reserved address space plus a guard page to elide any bounds checks at all. Smaller static memories will use similar bounds checks as dynamic memories.

Default

The default value for this property depends on the host platform. For 64-bit platforms there’s lots of address space available, so the default configured here is 4GB. WebAssembly linear memories currently max out at 4GB which means that on 64-bit platforms Wasmtime by default always uses a static memory. This, coupled with a sufficiently sized guard region, should produce the fastest JIT code on 64-bit platforms, but does require a large address space reservation for each wasm memory.

For 32-bit platforms this value defaults to 1GB. This means that wasm memories whose maximum size is less than 1GB will be allocated statically, otherwise they’ll be considered dynamic.

Static Memory and Pooled Instance Allocation

When using the pooling instance allocator memories are considered to always be static memories, they are never dynamic. This setting configures the size of linear memory to reserve for each memory in the pooling allocator.

Configures the size, in bytes, of the guard region used at the end of a static memory’s address space reservation.

Note: this value has important performance ramifications, be sure to understand what this value does before tweaking it and benchmarking.

All WebAssembly loads/stores are bounds-checked and generate a trap if they’re out-of-bounds. Loads and stores are often very performance critical, so we want the bounds check to be as fast as possible! Accelerating these memory accesses is the motivation for a guard after a memory allocation.

Memories (both static and dynamic) can be configured with a guard at the end of them which consists of unmapped virtual memory. This unmapped memory will trigger a memory access violation (e.g. segfault) if accessed. This allows JIT code to elide bounds checks if it can prove that an access, if out of bounds, would hit the guard region. This means that having such a guard of unmapped memory can remove the need for bounds checks in JIT code.

For the difference between static and dynamic memories, see the Config::static_memory_maximum_size.

How big should the guard be?

In general, like with configuring static_memory_maximum_size, you probably don’t want to change this value from the defaults. Otherwise, though, the size of the guard region affects the number of bounds checks needed for generated wasm code. More specifically, loads/stores with immediate offsets will generate bounds checks based on how big the guard page is.

For 32-bit memories a 4GB static memory is required to even start removing bounds checks. A 4GB guard size will guarantee that the module has zero bounds checks for memory accesses. A 2GB guard size will eliminate all bounds checks with an immediate offset less than 2GB. A guard size of zero means that all memory accesses will still have bounds checks.

Default

The default value for this property is 2GB on 64-bit platforms. This allows eliminating almost all bounds checks on loads/stores with an immediate offset of less than 2GB. On 32-bit platforms this defaults to 64KB.

Static vs Dynamic Guard Size

Note that for now the static memory guard size must be at least as large as the dynamic memory guard size, so configuring this property to be smaller than the dynamic memory guard size will have no effect.

Configures the size, in bytes, of the guard region used at the end of a dynamic memory’s address space reservation.

For the difference between static and dynamic memories, see the Config::static_memory_maximum_size

For more information about what a guard is, see the documentation on Config::static_memory_guard_size.

Note that the size of the guard region for dynamic memories is not super critical for performance. Making it reasonably-sized can improve generated code slightly, but for maximum performance you’ll want to lean towards static memories rather than dynamic anyway.

Also note that the dynamic memory guard size must be smaller than the static memory guard size, so if a large dynamic memory guard is specified then the static memory guard size will also be automatically increased.

Default

This value defaults to 64KB.

Configures the size, in bytes, of the extra virtual memory space reserved after a “dynamic” memory for growing into.

For the difference between static and dynamic memories, see the Config::static_memory_maximum_size

Dynamic memories can be relocated in the process’s virtual address space on growth and do not always reserve their entire space up-front. This means that a growth of the memory may require movement in the address space, which in the worst case can copy a large number of bytes from one region to another.

This setting configures how many bytes are reserved after the initial reservation for a dynamic memory for growing into. A value of 0 here means that no extra bytes are reserved and all calls to memory.grow will need to relocate the wasm linear memory (copying all the bytes). A value of 1 megabyte, however, means that memory.grow can allocate up to a megabyte of extra memory before the memory needs to be moved in linear memory.

Note that this is a currently simple heuristic for optimizing the growth of dynamic memories, primarily implemented for the memory64 propsal where all memories are currently “dynamic”. This is unlikely to be a one-size-fits-all style approach and if you’re an embedder running into issues with dynamic memories and growth and are interested in having other growth strategies available here please feel free to open an issue on the Wasmtime repository!

Default

For 64-bit platforms this defaults to 2GB, and for 32-bit platforms this defaults to 1MB.

Indicates whether a guard region is present before allocations of linear memory.

Guard regions before linear memories are never used during normal operation of WebAssembly modules, even if they have out-of-bounds loads. The only purpose for a preceding guard region in linear memory is extra protection against possible bugs in code generators like Cranelift. This setting does not affect performance in any way, but will result in larger virtual memory reservations for linear memories (it won’t actually ever use more memory, just use more of the address space).

The size of the guard region before linear memory is the same as the guard size that comes after linear memory, which is configured by Config::static_memory_guard_size and Config::dynamic_memory_guard_size.

Default

This value defaults to true.

Configure the version information used in serialized and deserialzied crate::Modules. This effects the behavior of crate::Module::serialize(), as well as crate::Module::deserialize() and related functions.

The default strategy is to use the wasmtime crate’s Cargo package version.

Configure wether wasmtime should compile a module using multiple threads.

Disabling this will result in a single thread being used to compile the wasm bytecode.

By default parallel compilation is enabled.

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