Add a method on Store to draw an MMU-based epoch to a close.

Keep the guts of the page-protecting operation on `VMStoreContext` next to where the page is mapped and unmapped.

Author: erikrose

crates/wasmtime/src/runtime/store.rs

@@ -1030,6 +1030,12 @@ impl<T> Store<T> {
     ) {
         self.inner.epoch_deadline_callback(Box::new(callback));
     }
+
+    /// Trigger the ending of the current "epoch" as soon as possible, if using
+    /// [`Config::epoch_interruption_via_mmu()`]. Panic if not.
+    pub fn end_epoch_via_mmu(&mut self) {
+        self.inner.inner.vm_store_context.protect_interrupt_page()
+    }
 }
 
 impl<'a, T> StoreContext<'a, T> {

crates/wasmtime/src/runtime/vm/vmcontext.rs

@@ -16,7 +16,7 @@ use core::mem::{self, MaybeUninit};
 use core::ops::Range;
 use core::ptr::{self, NonNull};
 use core::sync::atomic::{AtomicUsize, Ordering};
-use rustix::mm::{MapFlags, ProtFlags, mmap_anonymous, munmap};
+use rustix::mm::{MapFlags, MprotectFlags, ProtFlags, mmap_anonymous, mprotect, munmap};
 use rustix::param::page_size;
 use wasmtime_environ::{
     BuiltinFunctionIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex,
@@ -1227,6 +1227,20 @@ impl VMStoreContext {
         ret
     }
 
+    /// Twiddles MMU access control bits such that reads from the interrupt page
+    /// will cause a segfault. This effectively ends the epoch, as running wasm
+    /// will soon execute such reads.
+    pub fn protect_interrupt_page(&self) {
+        if let Some(page_ptr) = self.epoch_interrupt_page_ptr {
+            unsafe {
+                mprotect(page_ptr.as_ptr(), page_size(), MprotectFlags::empty())
+                    .expect("any error from mprotect is a programming error")
+            }
+        } else {
+            panic!("called protect_interrupt_page though epoch-interruption-via-mmu was not on")
+        }
+    }
+
     /// Disposes of any allocated epoch interrupt page. This must be called if
     /// [`VMStoreContext::with_interrupt_page()`] was used to construct this
     /// instance, lest we leak a page.