vm: Adapt qemu command to ARM.

* gnu/build/vm.scm (load-in-linux-vm): New argument #:target-arm32.
Use it to adapt command for qemu-system-arm.  This implies to choose a
machine ("virt"), use the correct console port "ttyAMA0", disable KVM use
that is buggy on some ARM boards (Odroid XU4 for example) and use user mode
network stack instead of NIC. Gather all those options in a new variable
"arch-specific-flags".
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Pass
to load-in-linux-vm "#:target-arm32?" argument.

gnu/build/vm.scm

@@ -77,6 +77,7 @@
                            linux initrd
                            make-disk-image?
                            single-file-output?
+                           target-arm32?
                            (disk-image-size (* 100 (expt 2 20)))
                            (disk-image-format "qcow2")
                            (references-graphs '()))
@@ -91,6 +92,31 @@ access it via /dev/hda.
 
 REFERENCES-GRAPHS can specify a list of reference-graph files as produced by
 the #:references-graphs parameter of 'derivation'."
+
+  (define arch-specific-flags
+    `(;; On ARM, a machine has to be specified. Use "virt" machine to avoid
+      ;; hardware limits imposed by other machines.
+      ,@(if target-arm32? '("-M" "virt") '())
+
+      ;; Only enable kvm if we see /dev/kvm exists.  This allows users without
+      ;; hardware virtualization to still use these commands.  KVM support is
+      ;; still buggy on some ARM32 boards. Do not use it even if available.
+      ,@(if (and (file-exists? "/dev/kvm")
+                 (not target-arm32?))
+            '("-enable-kvm")
+            '())
+      "-append"
+      ;; The serial port name differs between emulated architectures/machines.
+      ,@(if target-arm32?
+            `(,(string-append "console=ttyAMA0 --load=" builder))
+            `(,(string-append "console=ttyS0 --load=" builder)))
+      ;; NIC is not supported on ARM "virt" machine, so use a user mode
+      ;; network stack instead.
+      ,@(if target-arm32?
+            '("-device" "virtio-net-pci,netdev=mynet"
+              "-netdev" "user,id=mynet")
+            '("-net" "nic,model=virtio"))))
+
   (when make-disk-image?
     (format #t "creating ~a image of ~,2f MiB...~%"
             disk-image-format (/ disk-image-size (expt 2 20)))
@@ -113,7 +139,6 @@ the #:references-graphs parameter of 'derivation'."
   (unless (zero?
            (apply system* qemu "-nographic" "-no-reboot"
                   "-m" (number->string memory-size)
-                  "-net" "nic,model=virtio"
                   "-virtfs"
                   (string-append "local,id=store_dev,path="
                                  (%store-directory)
@@ -132,12 +157,7 @@ the #:references-graphs parameter of 'derivation'."
                                                   ",format=" disk-image-format
                                                   ",id=myhd"))
                        '())
-                   ;; Only enable kvm if we see /dev/kvm exists.
+                   arch-specific-flags)))
-                   ;; This allows users without hardware virtualization to still
-                   ;; use these commands.
-                   (if (file-exists? "/dev/kvm")
-                       '("-enable-kvm")
-                       '()))))
     (error "qemu failed" qemu))
 
   ;; When MAKE-DISK-IMAGE? is true, the image is in OUTPUT already.

gnu/system/vm.scm

@@ -175,6 +175,10 @@ made available under the /xchg CIFS share."
                                 #:memory-size #$memory-size
                                 #:make-disk-image? #$make-disk-image?
                                 #:single-file-output? #$single-file-output?
+                                ;; FIXME: ‘target-arm32?’ may not operate on
+                                ;; the right system/target values.  Rewrite
+                                ;; using ‘let-system’ when available.
+                                #:target-arm32? #$(target-arm32?)
                                 #:disk-image-format #$disk-image-format
                                 #:disk-image-size size
                                 #:references-graphs graphs)))))