machine/linux: Use memory-backend-memfd by default

This is prep for using virtiofsd; it has no real
impact otherwise.

Signed-off-by: Colin Walters <walters@verbum.org>

pkg/machine/qemu/command/command.go

@@ -35,8 +35,10 @@ func NewQemuBuilder(binary string, options []string) QemuCmd {
 
 // SetMemory adds the specified amount of memory for the machine
 func (q *QemuCmd) SetMemory(m strongunits.MiB) {
-	// qemu accepts the memory in MiB
-	*q = append(*q, "-m", strconv.FormatUint(uint64(m), 10))
+	serializedMem := strconv.FormatUint(uint64(m), 10)
+	// In order to use virtiofsd, we must enable shared memory
+	*q = append(*q, "-object", fmt.Sprintf("memory-backend-memfd,id=mem,size=%sM,share=on", serializedMem))
+	*q = append(*q, "-m", serializedMem)
 }
 
 // SetCPUs adds the number of CPUs the machine will have

pkg/machine/qemu/options_linux_amd64.go

@@ -10,6 +10,7 @@ func (q *QEMUStubber) addArchOptions(_ *setNewMachineCMDOpts) []string {
 	opts := []string{
 		"-accel", "kvm",
 		"-cpu", "host",
+		"-M", "memory-backend=mem",
 	}
 	return opts
 }

pkg/machine/qemu/options_linux_arm64.go

@@ -16,7 +16,7 @@ func (q *QEMUStubber) addArchOptions(_ *setNewMachineCMDOpts) []string {
 	opts := []string{
 		"-accel", "kvm",
 		"-cpu", "host",
-		"-M", "virt,gic-version=max",
+		"-M", "virt,gic-version=max,memory-backend=mem",
 		"-bios", getQemuUefiFile("QEMU_EFI.fd"),
 	}
 	return opts