set vhost on when vhost-net module is enabled

lisa/tools/qemu.py

@@ -10,7 +10,7 @@
 
 from lisa.executable import Tool
 from lisa.operating_system import Fedora, Posix, Redhat
-from lisa.tools import Ip, Kill, Lscpu, Lsmod, Pgrep
+from lisa.tools import Ip, KernelConfig, Kill, Lscpu, Lsmod, Pgrep
 from lisa.tools.lscpu import CpuType
 from lisa.util import LisaException, SkippedException, get_matched_str
 
@@ -104,9 +104,16 @@ def create_vm(
                     f"-device {nic_model},netdev=nettap{self.interface_count},"
                     f"mac={random_mac_address} "
                 )
-                cmd += (
-                    f"-netdev tap,id=nettap{self.interface_count},vhost=on,script=no "
-                )
+
+                base_cmd = f"-netdev tap,id=nettap{self.interface_count},script=no"
+
+                # vhost-net is a kernel module that accelerates virtio-net performance
+                # by offloading packet processing from QEMU's user space to the host
+                # kernel reducing latency and CPU overhead
+                if self.node.tools[KernelConfig].is_enabled("CONFIG_VHOST_NET"):
+                    base_cmd += ",vhost=on"
+
+                cmd += base_cmd + " "
                 added_taps.append(f"tap{self.interface_count}")
                 self.interface_count += 1