diff --git a/builder/qemu/builder.go b/builder/qemu/builder.go
index bce0f99d1..60bdf9fe7 100644
--- a/builder/qemu/builder.go
+++ b/builder/qemu/builder.go
@@ -92,15 +92,15 @@ type config struct {
 	tpl             *packer.ConfigTemplate
 }
 
-func (b *Builder) Prepare(raws ...interface{}) error {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	md, err := common.DecodeConfig(&b.config, raws...)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	b.config.tpl, err = packer.NewConfigTemplate()
 	if err != nil {
-		return err
+		return nil, err
 	}
 	b.config.tpl.UserVars = b.config.PackerUserVars
 
@@ -361,10 +361,10 @@ func (b *Builder) Prepare(raws ...interface{}) error {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return errs
+		return nil, errs
 	}
 
-	return nil
+	return nil, nil
 }
 
 func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packer.Artifact, error) {
diff --git a/builder/qemu/builder_test.go b/builder/qemu/builder_test.go
index 46b09891c..4fb4338a9 100644
--- a/builder/qemu/builder_test.go
+++ b/builder/qemu/builder_test.go
@@ -59,7 +59,10 @@ func TestBuilder_ImplementsBuilder(t *testing.T) {
 func TestBuilderPrepare_Defaults(t *testing.T) {
 	var b Builder
 	config := testConfig()
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -95,7 +98,10 @@ func TestBuilderPrepare_BootWait(t *testing.T) {
 
 	// Test a default boot_wait
 	delete(config, "boot_wait")
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("err: %s", err)
 	}
@@ -106,7 +112,10 @@ func TestBuilderPrepare_BootWait(t *testing.T) {
 
 	// Test with a bad boot_wait
 	config["boot_wait"] = "this is not good"
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -114,7 +123,10 @@ func TestBuilderPrepare_BootWait(t *testing.T) {
 	// Test with a good one
 	config["boot_wait"] = "5s"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -125,7 +137,10 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "disk_size")
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("bad err: %s", err)
 	}
@@ -136,7 +151,10 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 
 	config["disk_size"] = 60000
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -151,7 +169,10 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "floppy_files")
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("bad err: %s", err)
 	}
@@ -162,7 +183,10 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 
 	config["floppy_files"] = []string{"foo", "bar"}
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -180,7 +204,10 @@ func TestBuilderPrepare_HTTPPort(t *testing.T) {
 	// Bad
 	config["http_port_min"] = 1000
 	config["http_port_max"] = 500
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -188,7 +215,10 @@ func TestBuilderPrepare_HTTPPort(t *testing.T) {
 	// Bad
 	config["http_port_min"] = -500
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -197,7 +227,10 @@ func TestBuilderPrepare_HTTPPort(t *testing.T) {
 	config["http_port_min"] = 500
 	config["http_port_max"] = 1000
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -209,7 +242,10 @@ func TestBuilderPrepare_Format(t *testing.T) {
 
 	// Bad
 	config["format"] = "illegal value"
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -217,7 +253,10 @@ func TestBuilderPrepare_Format(t *testing.T) {
 	// Good
 	config["format"] = "qcow2"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -225,7 +264,10 @@ func TestBuilderPrepare_Format(t *testing.T) {
 	// Good
 	config["format"] = "raw"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -237,7 +279,10 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -249,7 +294,10 @@ func TestBuilderPrepare_ISOChecksum(t *testing.T) {
 
 	// Test bad
 	config["iso_checksum"] = ""
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -257,7 +305,10 @@ func TestBuilderPrepare_ISOChecksum(t *testing.T) {
 	// Test good
 	config["iso_checksum"] = "FOo"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -273,7 +324,10 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 
 	// Test bad
 	config["iso_checksum_type"] = ""
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -281,7 +335,10 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 	// Test good
 	config["iso_checksum_type"] = "mD5"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -293,7 +350,10 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 	// Test unknown
 	config["iso_checksum_type"] = "fake"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -308,7 +368,10 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	// Test both epty
 	config["iso_url"] = ""
 	b = Builder{}
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -316,7 +379,10 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	// Test iso_url set
 	config["iso_url"] = "http://www.packer.io"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Errorf("should not have error: %s", err)
 	}
@@ -330,7 +396,10 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	config["iso_url"] = "http://www.packer.io"
 	config["iso_urls"] = []string{"http://www.packer.io"}
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -343,7 +412,10 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	}
 
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Errorf("should not have error: %s", err)
 	}
@@ -370,7 +442,10 @@ func TestBuilderPrepare_OutputDir(t *testing.T) {
 
 	config["output_directory"] = dir
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -378,7 +453,10 @@ func TestBuilderPrepare_OutputDir(t *testing.T) {
 	// Test with a good one
 	config["output_directory"] = "i-hope-i-dont-exist"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -390,7 +468,10 @@ func TestBuilderPrepare_ShutdownTimeout(t *testing.T) {
 
 	// Test with a bad value
 	config["shutdown_timeout"] = "this is not good"
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -398,7 +479,10 @@ func TestBuilderPrepare_ShutdownTimeout(t *testing.T) {
 	// Test with a good one
 	config["shutdown_timeout"] = "5s"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -412,7 +496,10 @@ func TestBuilderPrepare_SSHHostPort(t *testing.T) {
 	config["ssh_host_port_min"] = 1000
 	config["ssh_host_port_max"] = 500
 	b = Builder{}
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -420,7 +507,10 @@ func TestBuilderPrepare_SSHHostPort(t *testing.T) {
 	// Bad
 	config["ssh_host_port_min"] = -500
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -429,7 +519,10 @@ func TestBuilderPrepare_SSHHostPort(t *testing.T) {
 	config["ssh_host_port_min"] = 500
 	config["ssh_host_port_max"] = 1000
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -441,14 +534,20 @@ func TestBuilderPrepare_sshKeyPath(t *testing.T) {
 
 	config["ssh_key_path"] = ""
 	b = Builder{}
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
 
 	config["ssh_key_path"] = "/i/dont/exist"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -467,7 +566,10 @@ func TestBuilderPrepare_sshKeyPath(t *testing.T) {
 
 	config["ssh_key_path"] = tf.Name()
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -478,7 +580,10 @@ func TestBuilderPrepare_sshKeyPath(t *testing.T) {
 	tf.Write([]byte(testPem))
 	config["ssh_key_path"] = tf.Name()
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("err: %s", err)
 	}
@@ -490,14 +595,20 @@ func TestBuilderPrepare_SSHUser(t *testing.T) {
 
 	config["ssh_username"] = ""
 	b = Builder{}
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
 
 	config["ssh_username"] = "exists"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -509,7 +620,10 @@ func TestBuilderPrepare_SSHWaitTimeout(t *testing.T) {
 
 	// Test a default boot_wait
 	delete(config, "ssh_wait_timeout")
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("err: %s", err)
 	}
@@ -521,7 +635,10 @@ func TestBuilderPrepare_SSHWaitTimeout(t *testing.T) {
 	// Test with a bad value
 	config["ssh_wait_timeout"] = "this is not good"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err == nil {
 		t.Fatal("should have error")
 	}
@@ -529,7 +646,10 @@ func TestBuilderPrepare_SSHWaitTimeout(t *testing.T) {
 	// Test with a good one
 	config["ssh_wait_timeout"] = "5s"
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}
@@ -541,7 +661,10 @@ func TestBuilderPrepare_QemuArgs(t *testing.T) {
 
 	// Test with empty
 	delete(config, "qemuargs")
-	err := b.Prepare(config)
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("err: %s", err)
 	}
@@ -556,7 +679,10 @@ func TestBuilderPrepare_QemuArgs(t *testing.T) {
 	}
 
 	b = Builder{}
-	err = b.Prepare(config)
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
 	if err != nil {
 		t.Fatalf("should not have error: %s", err)
 	}