diff --git a/tools/xen/functions b/tools/xen/functions
index 93f3413..bc0c515 100644
--- a/tools/xen/functions
+++ b/tools/xen/functions
@@ -294,6 +294,18 @@
     # Assert ithas a numeric nonzero value
     expr "$cpu_count" + 0
 
+    # 8 VCPUs should be enough for devstack VM; avoid using too
+    # many VCPUs:
+    # 1. too many VCPUs may trigger a kernel bug which result VM
+    #    not able to boot:
+    #    https://kernel.googlesource.com/pub/scm/linux/kernel/git/wsa/linux/+/e2e004acc7cbe3c531e752a270a74e95cde3ea48
+    # 2. The remaining CPUs can be used for other purpose:
+    #    e.g. boot test VMs.
+    MAX_VCPUS=8
+    if [ $cpu_count -ge $MAX_VCPUS ]; then
+        cpu_count=$MAX_VCPUS
+    fi
+
     xe vm-param-set uuid=$vm VCPUs-max=$cpu_count
     xe vm-param-set uuid=$vm VCPUs-at-startup=$cpu_count
 }