Add comments explaining why nullptr is passed for dev arg

sycl/source/detail/buffer_impl.cpp

@@ -84,6 +84,9 @@ buffer_impl::getNativeVector(backend BackendName) const {
     }
 
     pi_native_handle Handle;
+    // When doing buffer interop we don't know what device the memory should be
+    // resident on, so pass nullptr for Device param. Buffer interop may not be
+    // supported by all backends.
     Plugin->call<PiApiKind::piextMemGetNativeHandle>(NativeMem, /*Dev*/ nullptr,
                                                      &Handle);
     Handles.push_back(Handle);

sycl/source/detail/memory_manager.cpp

@@ -144,6 +144,9 @@ void memBufferCreateHelper(const PluginPtr &Plugin, pi_context Ctx,
       // Always use call_nocheck here, because call may throw an exception,
       // and this lambda will be called from destructor, which in combination
       // rewards us with UB.
+      // When doing buffer interop we don't know what device the memory should
+      // be resident on, so pass nullptr for Device param. Buffer interop may
+      // not be supported by all backends.
       Plugin->call_nocheck<PiApiKind::piextMemGetNativeHandle>(
           *RetMem, /*Dev*/ nullptr, &Ptr);
       emitMemAllocEndTrace(MemObjID, (uintptr_t)(Ptr), Size, 0 /* guard zone */,
@@ -168,6 +171,9 @@ void memReleaseHelper(const PluginPtr &Plugin, pi_mem Mem) {
   // Do not make unnecessary PI calls without instrumentation enabled
   if (xptiTraceEnabled()) {
     pi_native_handle PtrHandle = 0;
+    // When doing buffer interop we don't know what device the memory should be
+    // resident on, so pass nullptr for Device param. Buffer interop may not be
+    // supported by all backends.
     Plugin->call<PiApiKind::piextMemGetNativeHandle>(Mem, /*Dev*/ nullptr,
                                                      &PtrHandle);
     Ptr = (uintptr_t)(PtrHandle);