Merge branch 'yoctocloud-accelerator'

builder/qemu/builder.go

@@ -154,8 +154,20 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		if runtime.GOOS == "windows" {
 			b.config.Accelerator = "tcg"
 		} else {
-			b.config.Accelerator = "kvm"
+			// /dev/kvm is a kernel module that may be loaded if kvm is
+			// installed and the host supports VT-x extensions. To make sure
+			// this will actually work we need to os.Open() it. If os.Open fails
+			// the kernel module was not installed or loaded correctly.
+			if fp, err := os.Open("/dev/kvm"); err != nil {
+				b.config.Accelerator = "tcg"
+			} else {
+				fp.Close()
+				b.config.Accelerator = "kvm"
+			}
 		}
+		log.Printf("use detected accelerator: %s", b.config.Accelerator)
+	} else {
+		log.Printf("use specified accelerator: %s", b.config.Accelerator)
 	}
 
 	if b.config.MachineType == "" {

website/source/docs/builders/qemu.html.md

@@ -108,9 +108,10 @@ builder.
 ### Optional:
 
 -   `accelerator` (string) - The accelerator type to use when running the VM.
-    This may have a value of either "none", "kvm", "tcg", or "xen" and you must
-    have that support in on the machine on which you run the builder. By default
-    "kvm" is used.
+    This may be `none`, `kvm`, `tcg`, or `xen`. The appropriate software must
+    already been installed on your build machine to use the accelerator you
+    specified. When no accelerator is specified, Packer will try to use `kvm`
+    if it is available but will default to `tcg` otherwise.
 
 -   `boot_command` (array of strings) - This is an array of commands to type
     when the virtual machine is first booted. The goal of these commands should