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Delete historical interruptable support in Wasmtime (#3925)

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* Delete historical interruptable support in Wasmtime

This commit removes the `Config::interruptable` configuration along with
the `InterruptHandle` type from the `wasmtime` crate. The original
support for adding interruption to WebAssembly was added pretty early on
in the history of Wasmtime when there was no other method to prevent an
infinite loop from the host. Nowadays, however, there are alternative
methods for interruption such as fuel or epoch-based interruption.

One of the major downsides of `Config::interruptable` is that even when
it's not enabled it forces an atomic swap to happen when entering
WebAssembly code. This technically could be a non-atomic swap if the
configuration option isn't enabled but that produces even more branch-y
code on entry into WebAssembly which is already something we try to
optimize. Calling into WebAssembly is on the order of a dozens of
nanoseconds at this time and an atomic swap, even uncontended, can add
up to 5ns on some platforms.

The main goal of this PR is to remove this atomic swap on entry into
WebAssembly. This is done by removing the `Config::interruptable` field
entirely, moving all existing consumers to epochs instead which are
suitable for the same purposes. This means that the stack overflow check
is no longer entangled with the interruption check and perhaps one day
we could continue to optimize that further as well.

Some consequences of this change are:

* Epochs are now the only method of remote-thread interruption.
* There are no more Wasmtime traps that produces the `Interrupted` trap
  code, although we may wish to move future traps to this so I left it
  in place.
* The C API support for interrupt handles was also removed and bindings
  for epoch methods were added.
* Function-entry checks for interruption are a tiny bit less efficient
  since one check is performed for the stack limit and a second is
  performed for the epoch as opposed to the `Config::interruptable`
  style of bundling the stack limit and the interrupt check in one. It's
  expected though that this is likely to not really be measurable.
* The old `VMInterrupts` structure is renamed to `VMRuntimeLimits`.
This commit is contained in:
Alex Crichton
2022-03-14 15:25:11 -05:00
committed by GitHub
parent 62a6a7ab6c
commit c22033bf93
33 changed files with 293 additions and 589 deletions
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78
crates/cranelift/src/func_environ.rs
View File

@@ -17,7 +17,7 @@ use std::mem;
use wasmparser::Operator;
use wasmtime_environ::{
BuiltinFunctionIndex, MemoryPlan, MemoryStyle, Module, ModuleTranslation, TableStyle, Tunables,
TypeTables, VMOffsets, INTERRUPTED, WASM_PAGE_SIZE,
TypeTables, VMOffsets, WASM_PAGE_SIZE,
};
use wasmtime_environ::{FUNCREF_INIT_BIT, FUNCREF_MASK};
@@ -129,17 +129,17 @@ pub struct FuncEnvironment<'module_environment> {
/// A function-local variable which stores the cached value of the amount of
/// fuel remaining to execute. If used this is modified frequently so it's
/// stored locally as a variable instead of always referenced from the field
/// in `*const VMInterrupts`
/// in `*const VMRuntimeLimits`
fuel_var: cranelift_frontend::Variable,
/// A function-local variable which caches the value of `*const
/// VMInterrupts` for this function's vmctx argument. This pointer is stored
/// VMRuntimeLimits` for this function's vmctx argument. This pointer is stored
/// in the vmctx itself, but never changes for the lifetime of the function,
/// so if we load it up front we can continue to use it throughout.
vminterrupts_ptr: cranelift_frontend::Variable,
vmruntime_limits_ptr: cranelift_frontend::Variable,
/// A cached epoch deadline value, when performing epoch-based
/// interruption. Loaded from `VMInterrupts` and reloaded after
/// interruption. Loaded from `VMRuntimeLimits` and reloaded after
/// any yield.
epoch_deadline_var: cranelift_frontend::Variable,
@@ -182,7 +182,7 @@ impl<'module_environment> FuncEnvironment<'module_environment> {
fuel_var: Variable::new(0),
epoch_deadline_var: Variable::new(0),
epoch_ptr_var: Variable::new(0),
vminterrupts_ptr: Variable::new(0),
vmruntime_limits_ptr: Variable::new(0),
// Start with at least one fuel being consumed because even empty
// functions should consume at least some fuel.
@@ -344,27 +344,27 @@ impl<'module_environment> FuncEnvironment<'module_environment> {
}
}
fn declare_vminterrupts_ptr(&mut self, builder: &mut FunctionBuilder<'_>) {
// We load the `*const VMInterrupts` value stored within vmctx at the
fn declare_vmruntime_limits_ptr(&mut self, builder: &mut FunctionBuilder<'_>) {
// We load the `*const VMRuntimeLimits` value stored within vmctx at the
// head of the function and reuse the same value across the entire
// function. This is possible since we know that the pointer never
// changes for the lifetime of the function.
let pointer_type = self.pointer_type();
builder.declare_var(self.vminterrupts_ptr, pointer_type);
builder.declare_var(self.vmruntime_limits_ptr, pointer_type);
let vmctx = self.vmctx(builder.func);
let base = builder.ins().global_value(pointer_type, vmctx);
let offset = i32::try_from(self.offsets.vmctx_interrupts()).unwrap();
let offset = i32::try_from(self.offsets.vmctx_runtime_limits()).unwrap();
let interrupt_ptr = builder
.ins()
.load(pointer_type, ir::MemFlags::trusted(), base, offset);
builder.def_var(self.vminterrupts_ptr, interrupt_ptr);
builder.def_var(self.vmruntime_limits_ptr, interrupt_ptr);
}
fn fuel_function_entry(&mut self, builder: &mut FunctionBuilder<'_>) {
// On function entry we load the amount of fuel into a function-local
// `self.fuel_var` to make fuel modifications fast locally. This cache
// is then periodically flushed to the Store-defined location in
// `VMInterrupts` later.
// `VMRuntimeLimits` later.
builder.declare_var(self.fuel_var, ir::types::I64);
self.fuel_load_into_var(builder);
self.fuel_check(builder);
@@ -412,13 +412,13 @@ impl<'module_environment> FuncEnvironment<'module_environment> {
match op {
// Exiting a function (via a return or unreachable) or otherwise
// entering a different function (via a call) means that we need to
// update the fuel consumption in `VMInterrupts` because we're
// update the fuel consumption in `VMRuntimeLimits` because we're
// about to move control out of this function itself and the fuel
// may need to be read.
//
// Before this we need to update the fuel counter from our own cost
// leading up to this function call, and then we can store
// `self.fuel_var` into `VMInterrupts`.
// `self.fuel_var` into `VMRuntimeLimits`.
Operator::Unreachable
| Operator::Return
| Operator::CallIndirect { .. }
@@ -502,7 +502,7 @@ impl<'module_environment> FuncEnvironment<'module_environment> {
builder.def_var(self.fuel_var, fuel);
}
/// Loads the fuel consumption value from `VMInterrupts` into `self.fuel_var`
/// Loads the fuel consumption value from `VMRuntimeLimits` into `self.fuel_var`
fn fuel_load_into_var(&mut self, builder: &mut FunctionBuilder<'_>) {
let (addr, offset) = self.fuel_addr_offset(builder);
let fuel = builder
@@ -512,7 +512,7 @@ impl<'module_environment> FuncEnvironment<'module_environment> {
}
/// Stores the fuel consumption value from `self.fuel_var` into
/// `VMInterrupts`.
/// `VMRuntimeLimits`.
fn fuel_save_from_var(&mut self, builder: &mut FunctionBuilder<'_>) {
let (addr, offset) = self.fuel_addr_offset(builder);
let fuel_consumed = builder.use_var(self.fuel_var);
@@ -522,14 +522,14 @@ impl<'module_environment> FuncEnvironment<'module_environment> {
}
/// Returns the `(address, offset)` of the fuel consumption within
/// `VMInterrupts`, used to perform loads/stores later.
/// `VMRuntimeLimits`, used to perform loads/stores later.
fn fuel_addr_offset(
&mut self,
builder: &mut FunctionBuilder<'_>,
) -> (ir::Value, ir::immediates::Offset32) {
(
builder.use_var(self.vminterrupts_ptr),
i32::from(self.offsets.vminterrupts_fuel_consumed()).into(),
builder.use_var(self.vmruntime_limits_ptr),
i32::from(self.offsets.vmruntime_limits_fuel_consumed()).into(),
)
}
@@ -628,12 +628,12 @@ impl<'module_environment> FuncEnvironment<'module_environment> {
}
fn epoch_load_deadline_into_var(&mut self, builder: &mut FunctionBuilder<'_>) {
let interrupts = builder.use_var(self.vminterrupts_ptr);
let interrupts = builder.use_var(self.vmruntime_limits_ptr);
let deadline = builder.ins().load(
ir::types::I64,
ir::MemFlags::trusted(),
interrupts,
ir::immediates::Offset32::new(self.offsets.vminterupts_epoch_deadline() as i32),
ir::immediates::Offset32::new(self.offsets.vmruntime_limits_epoch_deadline() as i32),
);
builder.def_var(self.epoch_deadline_var, deadline);
}
@@ -824,7 +824,7 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
}
fn after_locals(&mut self, num_locals: usize) {
self.vminterrupts_ptr = Variable::new(num_locals);
self.vmruntime_limits_ptr = Variable::new(num_locals);
self.fuel_var = Variable::new(num_locals + 1);
self.epoch_deadline_var = Variable::new(num_locals + 2);
self.epoch_ptr_var = Variable::new(num_locals + 3);
@@ -1976,31 +1976,6 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
}
fn translate_loop_header(&mut self, builder: &mut FunctionBuilder) -> WasmResult<()> {
// If enabled check the interrupt flag to prevent long or infinite
// loops.
//
// For more information about this see comments in
// `crates/environ/src/cranelift.rs`
if self.tunables.interruptable {
let pointer_type = self.pointer_type();
let interrupt_ptr = builder.use_var(self.vminterrupts_ptr);
let interrupt = builder.ins().load(
pointer_type,
ir::MemFlags::trusted(),
interrupt_ptr,
i32::from(self.offsets.vminterrupts_stack_limit()),
);
// Note that the cast to `isize` happens first to allow sign-extension,
// if necessary, to `i64`.
let interrupted_sentinel = builder
.ins()
.iconst(pointer_type, INTERRUPTED as isize as i64);
let cmp = builder
.ins()
.icmp(IntCC::Equal, interrupt, interrupted_sentinel);
builder.ins().trapnz(cmp, ir::TrapCode::Interrupt);
}
// Additionally if enabled check how much fuel we have remaining to see
// if we've run out by this point.
if self.tunables.consume_fuel {
@@ -2045,13 +2020,10 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
builder: &mut FunctionBuilder,
_state: &FuncTranslationState,
) -> WasmResult<()> {
// If the `vminterrupts_ptr` variable will get used then we initialize
// If the `vmruntime_limits_ptr` variable will get used then we initialize
// it here.
if self.tunables.consume_fuel
|| self.tunables.interruptable
|| self.tunables.epoch_interruption
{
self.declare_vminterrupts_ptr(builder);
if self.tunables.consume_fuel || self.tunables.epoch_interruption {
self.declare_vmruntime_limits_ptr(builder);
}
// Additionally we initialize `fuel_var` if it will get used.
if self.tunables.consume_fuel {