fix(test_rng): ensure virtio_rng is the current device used by hwrng

ARM Uvm with 6.1 kernel have an additional SMCCC TRNG device which is
paravirtualized by KVM. We need to ensure we are testing the virtio-rng
device, which is the one picked up by default by the kernel, if
available.

Signed-off-by: Riccardo Mancini <[email protected]>

Author: Manciukic

tests/integration_tests/functional/test_rng.py

@@ -32,6 +32,21 @@ def list_rng_available(ssh_connection: SSHConnection) -> list[str]:
     )
 
 
+def get_rng_current(ssh_connection: SSHConnection) -> str:
+    """Returns the current rng device used by hwrng"""
+    return ssh_connection.check_output(
+        "cat /sys/class/misc/hw_random/rng_current"
+    ).stdout.strip()
+
+
+def assert_virtio_rng_is_current_hwrng_device(ssh_connection: SSHConnection):
+    """Asserts that virtio_rng is the current device used by hwrng"""
+    # we expect something like virtio_rng.0
+    assert get_rng_current(ssh_connection).startswith(
+        "virtio_rng"
+    ), "virtio_rng device should be the current used by hwrng"
+
+
 def test_rng_not_present(uvm_nano):
     """
     Test a guest microVM *without* an entropy device and ensure that
@@ -54,6 +69,7 @@ def test_rng_present(uvm_with_rng):
     """
 
     vm = uvm_with_rng
+    assert_virtio_rng_is_current_hwrng_device(vm.ssh)
     check_entropy(vm.ssh)
 
 
@@ -64,13 +80,15 @@ def test_rng_snapshot(uvm_with_rng, microvm_factory):
     """
 
     vm = uvm_with_rng
+    assert_virtio_rng_is_current_hwrng_device(vm.ssh)
     check_entropy(vm.ssh)
     snapshot = vm.snapshot_full()
 
     new_vm = microvm_factory.build()
     new_vm.spawn()
     new_vm.restore_from_snapshot(snapshot, resume=True)
     new_vm.wait_for_up()
+    assert_virtio_rng_is_current_hwrng_device(new_vm.ssh)
     check_entropy(new_vm.ssh)
 
 
@@ -203,6 +221,7 @@ def test_rng_bw_rate_limiter(uvm_with_rng):
 
     expected_kbps = size / refill_time
 
+    assert_virtio_rng_is_current_hwrng_device(vm.ssh)
     # Check the rate limiter using a request size equal to the size
     # of the token bucket.
     _check_entropy_rate_limited(vm.ssh, size, expected_kbps)