Skip memfd tests when on qemu, due to differing madvise semantics.

crates/runtime/src/memfd.rs

@@ -594,6 +594,10 @@ mod test {
 
     #[test]
     fn instantiate_no_image() {
+        if skip_tests_due_to_qemu_madvise_semantics() {
+            return;
+        }
+
         // 4 MiB mmap'd area, not accessible
         let mut mmap = Mmap::accessible_reserved(0, 4 << 20).unwrap();
         // Create a MemFdSlot on top of it
@@ -626,6 +630,10 @@ mod test {
 
     #[test]
     fn instantiate_image() {
+        if skip_tests_due_to_qemu_madvise_semantics() {
+            return;
+        }
+
         // 4 MiB mmap'd area, not accessible
         let mut mmap = Mmap::accessible_reserved(0, 4 << 20).unwrap();
         // Create a MemFdSlot on top of it
@@ -669,4 +677,19 @@ mod test {
         let slice = mmap.as_slice();
         assert_eq!(&[1, 2, 3, 4], &slice[4096..4100]);
     }
+
+    /// qemu's madvise implementation does not implement the
+    /// "flash-reset back to zero or CoW backing" semantics that Linux
+    /// does. Our CI setup uses qemu (in usermode-binary mode, not
+    /// whole-system mode) to run tests on aarch64 and s390x. We want
+    /// to skip these tests when under qemu, but not when someone is
+    /// developing natively on one of these architectures. So instead,
+    /// we dynamically detect an environment variable that our CI
+    /// setup sets.
+    ///
+    /// See `skip_pooling_allocator_tests()` in `tests/all/main.rs`
+    /// for more.
+    fn skip_tests_due_to_qemu_madvise_semantics() -> bool {
+        std::env::var("WASMTIME_TEST_NO_HOG_MEMORY").is_ok()
+    }
 }