vmm: Limit the maximum number of supported vCPUs to 32

The firecracker VMM targets to support small scale workloads, so limit
the maximum number of supported vCPUs to 32.

Signed-off-by: Liu Jiang <[email protected]>

Author: jiangliu

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Added
 
 - Documentation for development environment setup on AWS.
+- Limit the maximum supported vCPUs to 32.
 
 ### Changed
 - Log the app version when the `Logger` is initialized.

api_server/src/http_service.rs

@@ -1101,7 +1101,7 @@ mod tests {
     fn test_parse_machine_config_req() {
         let path = "/machine-config";
         let json = "{
-                \"vcpu_count\": 42,
+                \"vcpu_count\": 32,
                 \"mem_size_mib\": 1025,
                 \"ht_enabled\": true,
                 \"cpu_template\": \"T2\"
@@ -1118,7 +1118,7 @@ mod tests {
 
         // PUT
         let vm_config = VmConfig {
-            vcpu_count: Some(42),
+            vcpu_count: Some(32),
             mem_size_mib: Some(1025),
             ht_enabled: Some(true),
             cpu_template: Some(CpuFeaturesTemplate::T2),
@@ -1145,6 +1145,20 @@ mod tests {
             String::from("Empty request."),
         ));
         assert!(parse_machine_config_req(path, Method::Put, &Chunk::from("{}")) == expected_err);
+
+        // Error Case: cpu count exceeds limitation
+        let json = "{
+                \"vcpu_count\": 33,
+                \"mem_size_mib\": 1025,
+                \"ht_enabled\": true,
+                \"cpu_template\": \"T2\"
+              }";
+        let body: Chunk = Chunk::from(json);
+        if let Err(Error::SerdeJson(e)) = parse_machine_config_req(path, Method::Put, &body) {
+            assert!(e.is_data());
+        } else {
+            assert!(false);
+        }
     }
 
     #[test]

api_server/swagger/firecracker.yaml

@@ -430,6 +430,8 @@ definitions:
     properties:
       vcpu_count:
         type: integer
+        minimum: 1
+        maximum: 32
         description: Number of vCPUs (either 1 or an even number)
       mem_size_mib:
         type: integer

vmm/src/vmm_config/machine_config.rs

@@ -1,8 +1,13 @@
 // Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+use serde::{de, Deserialize};
 use std::fmt::{Display, Formatter, Result};
 
+/// Firecracker aims to support small scale workloads only, so limit the maximum
+/// vCPUs supported.
+pub const MAX_SUPPORTED_VCPUS: u8 = 32;
+
 /// Errors associated with configuring the microVM.
 #[derive(Debug, PartialEq)]
 pub enum VmConfigError {
@@ -38,7 +43,11 @@ impl Display for VmConfigError {
 #[serde(deny_unknown_fields)]
 pub struct VmConfig {
     /// Number of vcpu to start.
-    #[serde(skip_serializing_if = "Option::is_none")]
+    #[serde(
+        default,
+        skip_serializing_if = "Option::is_none",
+        deserialize_with = "validate_vcpu_num"
+    )]
     pub vcpu_count: Option<u8>,
     /// The memory size in MiB.
     #[serde(skip_serializing_if = "Option::is_none")]
@@ -62,6 +71,22 @@ impl Default for VmConfig {
     }
 }
 
+fn validate_vcpu_num<'de, D>(d: D) -> std::result::Result<Option<u8>, D::Error>
+where
+    D: de::Deserializer<'de>,
+{
+    let val = Option::<u8>::deserialize(d)?;
+    if let Some(ref value) = val {
+        if *value > MAX_SUPPORTED_VCPUS {
+            return Err(de::Error::invalid_value(
+                de::Unexpected::Unsigned(*value as u64),
+                &"number of vCPUs exceeds the maximum limitation",
+            ));
+        }
+    }
+    Ok(val)
+}
+
 /// Template types available for configuring the CPU features that map
 /// to EC2 instances.
 #[derive(Clone, Copy, Debug, Deserialize, PartialEq, Serialize)]