qemu: add file backed memory device support

It allows a caller to use a local file as the memory backend of the
guest, and it also allows the file backed memory device to be set shared
or not.

Signed-off-by: Peng Tao <bergwolf@gmail.com>

qemu/qemu.go

@@ -1126,6 +1126,10 @@ type Memory struct {
 	// MaxMem is the maximum amount of memory that can be made available
 	// to the guest through e.g. hot pluggable memory.
 	MaxMem string
+
+	// Path is the file path of the memory device. It points to a local
+	// file path used by FileBackedMem.
+	Path string
 }
 
 // Kernel is the guest kernel configuration structure.
@@ -1167,6 +1171,13 @@ type Knobs struct {
 	// MemPrealloc will allocate all the RAM upfront
 	MemPrealloc bool
 
+	// FileBackedMem requires Memory.Size and Memory.Path of the VM to
+	// be set.
+	FileBackedMem bool
+
+	// FileBackedMemShared will set the FileBackedMem device as shared.
+	FileBackedMemShared bool
+
 	// Mlock will control locking of memory
 	// Only active when Realtime is set to true
 	Mlock bool
@@ -1474,6 +1485,21 @@ func (config *Config) appendKnobs() {
 			config.qemuParams = append(config.qemuParams, "-device")
 			config.qemuParams = append(config.qemuParams, deviceMemParam)
 		}
+	} else if config.Knobs.FileBackedMem == true {
+		if config.Memory.Size != "" && config.Memory.Path != "" {
+			dimmName := "dimm1"
+			objMemParam := "memory-backend-file,id=" + dimmName + ",size=" + config.Memory.Size + ",mem-path=" + config.Memory.Path
+			if config.Knobs.FileBackedMemShared == true {
+				objMemParam += ",share=on"
+			}
+			numaMemParam := "node,memdev=" + dimmName
+
+			config.qemuParams = append(config.qemuParams, "-object")
+			config.qemuParams = append(config.qemuParams, objMemParam)
+
+			config.qemuParams = append(config.qemuParams, "-numa")
+			config.qemuParams = append(config.qemuParams, numaMemParam)
+		}
 	}
 
 	if config.Knobs.Realtime == true {

qemu/qemu_test.go

@@ -395,6 +395,8 @@ func TestAppendKnobsAllTrue(t *testing.T) {
 		NoGraphic:           true,
 		Daemonize:           true,
 		MemPrealloc:         true,
+		FileBackedMem:       true,
+		FileBackedMemShared: true,
 		Realtime:            true,
 		Mlock:               true,
 	}
@@ -409,6 +411,8 @@ func TestAppendKnobsAllFalse(t *testing.T) {
 		NoDefaults:          false,
 		NoGraphic:           false,
 		MemPrealloc:         false,
+		FileBackedMem:       false,
+		FileBackedMemShared: false,
 		Realtime:            false,
 		Mlock:               false,
 	}
@@ -435,6 +439,7 @@ func TestAppendMemory(t *testing.T) {
 		Size:   "2G",
 		Slots:  2,
 		MaxMem: "3G",
+		Path:   "",
 	}
 
 	testAppend(memory, memoryString, t)