feat: support setting default vcpu and memory size for vm

config/config.go

@@ -49,6 +49,12 @@ type Config struct {
 	CPUTemplate           string   `json:"cpu_template"`
 	LogLevels             []string `json:"log_levels"`
 	SmtEnabled            bool     `json:"smt_enabled"`
+	// DefaultVcpuCount is the default number of vCPUs for a microVM.
+	// If not specified (0), defaults to 1.
+	DefaultVcpuCount uint32 `json:"default_vcpu_count"`
+	// DefaultMemSizeMib is the default memory size in MiB for a microVM.
+	// Must be at least 1 MiB. If not specified (0), defaults to 128 MiB.
+	DefaultMemSizeMib uint32 `json:"default_mem_size_mib"`
 	// If a CreateVM call specifies no network interfaces and DefaultNetworkInterfaces is non-empty,
 	// the VM will default to using the network interfaces as specified here. This is especially
 	// useful when a CNI-based network interface is provided in DefaultNetworkInterfaces.

config/config.json.example

@@ -5,5 +5,7 @@
   "root_drive": "./vsock.img",
   "cpu_template": "T2",
   "log_levels": ["debug"],
-  "ht_enabled": false
+  "smt_enabled": false,
+  "default_vcpu_count": 1,
+  "default_mem_size_mib": 128
 }

config/config_test.go

@@ -76,3 +76,43 @@ func createTempConfig(t *testing.T, contents string) (string, func()) {
 	}
 	return configFile.Name(), func() { os.Remove(configFile.Name()) }
 }
+
+func TestLoadConfigDefaultVcpuAndMem(t *testing.T) {
+	testcases := []struct {
+		name          string
+		configContent string
+		expectedVcpu  uint32
+		expectedMem   uint32
+	}{
+		{
+			name:          "DefaultsWhenNotSpecified",
+			configContent: `{}`,
+			expectedVcpu:  0, // 0 means use hardcoded default
+			expectedMem:   0, // 0 means use hardcoded default
+		},
+		{
+			name:          "CustomVcpuAndMem",
+			configContent: `{"default_vcpu_count": 4, "default_mem_size_mib": 512}`,
+			expectedVcpu:  4,
+			expectedMem:   512,
+		},
+		{
+			name:          "LargeVcpuCount",
+			configContent: `{"default_vcpu_count": 32}`,
+			expectedVcpu:  32,
+			expectedMem:   0,
+		},
+	}
+
+	for _, tc := range testcases {
+		t.Run(tc.name, func(t *testing.T) {
+			configFile, cleanup := createTempConfig(t, tc.configContent)
+			defer cleanup()
+
+			cfg, err := LoadConfig(configFile)
+			assert.NoError(t, err)
+			assert.Equal(t, tc.expectedVcpu, cfg.DefaultVcpuCount)
+			assert.Equal(t, tc.expectedMem, cfg.DefaultMemSizeMib)
+		})
+	}
+}

runtime/helpers.go

@@ -31,10 +31,21 @@ const (
 )
 
 func machineConfigurationFromProto(cfg *config.Config, req *proto.FirecrackerMachineConfiguration) models.MachineConfiguration {
+	// Use config values if provided, otherwise fall back to hardcoded defaults
+	vcpuCount := int64(defaultCPUCount)
+	if cfg.DefaultVcpuCount > 0 {
+		vcpuCount = int64(cfg.DefaultVcpuCount)
+	}
+
+	memSizeMib := int64(defaultMemSizeMb)
+	if cfg.DefaultMemSizeMib > 0 {
+		memSizeMib = int64(cfg.DefaultMemSizeMib)
+	}
+
 	config := models.MachineConfiguration{
 		CPUTemplate: models.CPUTemplate(cfg.CPUTemplate),
-		VcpuCount:   firecracker.Int64(defaultCPUCount),
-		MemSizeMib:  firecracker.Int64(defaultMemSizeMb),
+		VcpuCount:   firecracker.Int64(vcpuCount),
+		MemSizeMib:  firecracker.Int64(memSizeMib),
 		Smt:         firecracker.Bool(cfg.SmtEnabled),
 	}
 

runtime/helpers_test.go

@@ -99,6 +99,69 @@ func TestMachineConfigurationFromProto(t *testing.T) {
 				Smt:         firecracker.Bool(true),
 			},
 		},
+		{
+			name: "ConfigDefaultVcpuAndMem",
+			config: &config.Config{
+				CPUTemplate:       "T2",
+				DefaultVcpuCount:  4,
+				DefaultMemSizeMib: 512,
+			},
+			proto: nil,
+			expectedMachineConfig: models.MachineConfiguration{
+				CPUTemplate: models.CPUTemplateT2,
+				VcpuCount:   firecracker.Int64(4),
+				MemSizeMib:  firecracker.Int64(512),
+				Smt:         firecracker.Bool(false),
+			},
+		},
+		{
+			name: "ConfigDefaultsWithEmptyProto",
+			config: &config.Config{
+				CPUTemplate:       "T2",
+				DefaultVcpuCount:  8,
+				DefaultMemSizeMib: 1024,
+			},
+			proto: &proto.FirecrackerMachineConfiguration{},
+			expectedMachineConfig: models.MachineConfiguration{
+				CPUTemplate: models.CPUTemplateT2,
+				VcpuCount:   firecracker.Int64(8),
+				MemSizeMib:  firecracker.Int64(1024),
+				Smt:         firecracker.Bool(false),
+			},
+		},
+		{
+			name: "ProtoOverridesConfigDefaults",
+			config: &config.Config{
+				CPUTemplate:       "T2",
+				DefaultVcpuCount:  4,
+				DefaultMemSizeMib: 512,
+			},
+			proto: &proto.FirecrackerMachineConfiguration{
+				VcpuCount:  16,
+				MemSizeMib: 2048,
+			},
+			expectedMachineConfig: models.MachineConfiguration{
+				CPUTemplate: models.CPUTemplateT2,
+				VcpuCount:   firecracker.Int64(16),
+				MemSizeMib:  firecracker.Int64(2048),
+				Smt:         firecracker.Bool(false),
+			},
+		},
+		{
+			name: "ZeroConfigDefaultsFallbackToHardcoded",
+			config: &config.Config{
+				CPUTemplate:       "T2",
+				DefaultVcpuCount:  0,
+				DefaultMemSizeMib: 0,
+			},
+			proto: nil,
+			expectedMachineConfig: models.MachineConfiguration{
+				CPUTemplate: models.CPUTemplateT2,
+				VcpuCount:   firecracker.Int64(defaultCPUCount),
+				MemSizeMib:  firecracker.Int64(defaultMemSizeMb),
+				Smt:         firecracker.Bool(false),
+			},
+		},
 	}
 
 	for _, tc := range testcases {