[SYCL] Fix reported P2P accessibility across platforms

sycl/source/device.cpp

@@ -236,6 +236,12 @@ void device::ext_oneapi_disable_peer_access(const device &peer) {
 
 bool device::ext_oneapi_can_access_peer(const device &peer,
                                         ext::oneapi::peer_access attr) {
+  // Peer access cannot be granted across platforms, but the handles could
+  // potentially mimic device handles from other adapters, so we need to avoid
+  // calling the adapters with handles from other platforms.
+  if (peer.get_platform() != get_platform())
+    return false;
+
   ur_device_handle_t Device = impl->getHandleRef();
   ur_device_handle_t Peer = peer.impl->getHandleRef();
 

sycl/test-e2e/USM/P2P/p2p_access_across_platforms.cpp

@@ -0,0 +1,28 @@
+// RUN:  %{build} -o %t.out
+// RUN:  %{run} %t.out
+
+// Tests that P2P access is not reported as possible across platforms.
+
+#include <cassert>
+#include <sycl/detail/core.hpp>
+#include <sycl/platform.hpp>
+
+int main() {
+  sycl::device D1;
+  sycl::device D2 = D1;
+  for (sycl::platform P : sycl::platform::get_platforms()) {
+    if (P != D1.get_platform() && !P.get_devices().empty()) {
+      D2 = P.get_devices()[0];
+      break;
+    }
+  }
+
+  if (D1 == D2) {
+    std::cout << "There are no devices from different platforms. Skipping."
+              << std::endl;
+    return 0;
+  }
+
+  assert(!D1.ext_oneapi_can_access_peer(D2));
+  return 0;
+}