qemu: Add maxcpus attribute to -smp

maxcpus is used to specify how many cpus a VM can have.
This attribute must be specified to enable the hotplugging CPUs capability,
otherwise the maximum number of CPU will be defined by the number of CPU
in -smp.

Signed-off-by: Julio Montes <julio.montes@intel.com>

qemu/qemu.go

@@ -980,6 +980,10 @@ type SMP struct {
 
 	// Sockets is the number of sockets made available to qemu.
 	Sockets uint32
+
+	// MaxCPUs is the maximum number of VCPUs that a VM can have.
+	// This value, if non-zero, MUST BE equal to or greater than CPUs
+	MaxCPUs uint32
 }
 
 // Memory is the guest memory configuration structure.
@@ -1203,7 +1207,7 @@ func (config *Config) appendMemory() {
 	}
 }
 
-func (config *Config) appendCPUs() {
+func (config *Config) appendCPUs() error {
 	if config.SMP.CPUs > 0 {
 		var SMPParams []string
 
@@ -1221,9 +1225,19 @@ func (config *Config) appendCPUs() {
 			SMPParams = append(SMPParams, fmt.Sprintf(",sockets=%d", config.SMP.Sockets))
 		}
 
+		if config.SMP.MaxCPUs > 0 {
+			if config.SMP.MaxCPUs < config.SMP.CPUs {
+				return fmt.Errorf("MaxCPUs %d must be equal to or greater than CPUs %d",
+					config.SMP.MaxCPUs, config.SMP.CPUs)
+			}
+			SMPParams = append(SMPParams, fmt.Sprintf(",maxcpus=%d", config.SMP.MaxCPUs))
+		}
+
 		config.qemuParams = append(config.qemuParams, "-smp")
 		config.qemuParams = append(config.qemuParams, strings.Join(SMPParams, ""))
 	}
+
+	return nil
 }
 
 func (config *Config) appendRTC() {
@@ -1360,7 +1374,6 @@ func LaunchQemu(config Config, logger QMPLog) (string, error) {
 	config.appendCPUModel()
 	config.appendQMPSockets()
 	config.appendMemory()
-	config.appendCPUs()
 	config.appendDevices()
 	config.appendRTC()
 	config.appendGlobalParam()
@@ -1369,6 +1382,10 @@ func LaunchQemu(config Config, logger QMPLog) (string, error) {
 	config.appendKernel()
 	config.appendBios()
 
+	if err := config.appendCPUs(); err != nil {
+		return "", err
+	}
+
 	return LaunchCustomQemu(config.Ctx, config.Path, config.qemuParams,
 		config.fds, nil, logger)
 }

qemu/qemu_test.go

@@ -52,7 +52,9 @@ func testAppend(structure interface{}, expected string, t *testing.T) {
 
 	case SMP:
 		config.SMP = s
-		config.appendCPUs()
+		if err := config.appendCPUs(); err != nil {
+			t.Fatalf("Unexpected error: %v\n", err)
+		}
 
 	case QMPSocket:
 		config.QMPSockets = []QMPSocket{s}
@@ -356,7 +358,7 @@ func TestAppendMemory(t *testing.T) {
 	testAppend(memory, memoryString, t)
 }
 
-var cpusString = "-smp 2,cores=1,threads=2,sockets=2"
+var cpusString = "-smp 2,cores=1,threads=2,sockets=2,maxcpus=6"
 
 func TestAppendCPUs(t *testing.T) {
 	smp := SMP{
@@ -364,11 +366,28 @@ func TestAppendCPUs(t *testing.T) {
 		Sockets: 2,
 		Cores:   1,
 		Threads: 2,
+		MaxCPUs: 6,
 	}
 
 	testAppend(smp, cpusString, t)
 }
 
+func TestFailToAppendCPUs(t *testing.T) {
+	config := Config{
+		SMP: SMP{
+			CPUs:    2,
+			Sockets: 2,
+			Cores:   1,
+			Threads: 2,
+			MaxCPUs: 1,
+		},
+	}
+
+	if err := config.appendCPUs(); err == nil {
+		t.Fatalf("Expected appendCPUs to fail")
+	}
+}
+
 var qmpSingleSocketServerString = "-qmp unix:cc-qmp,server,nowait"
 var qmpSingleSocketString = "-qmp unix:cc-qmp"