[PATCH] qemu: Add USB and memory balloon by default for aarch64/virt guests

[Andrea Bolognani <abologna@xxxxxxxxxx>]

Basically all existing guest types, regardless of the architectur,
get both a USB controller and a virtio memory balloon by default.

s390 guests are an exception, for the very good reason that they
don't support USB at all; the other exception is aarch64/virt
guests, but in the latter case isn't a compelling reason for them
to deviate from the widely adopted convention, especially since:

  * x86/q35 guests, which aarch64/virt guests are for the most
    part identical to, add these devices by default;
  * it's trivial to opt out of both default devices by setting
    model='none';
  * higher level applications such as Nova expect at least the
    USB controller to be present.

So add it by default for newly-defined guests. Existing guests
will, of course, be left unchanged.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1538637

Signed-off-by: Andrea Bolognani <abologna@xxxxxxxxxx>
---
 src/qemu/qemu_domain.c | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 6b4bd3cca..f1c3b3d1e 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -2630,7 +2630,8 @@ qemuDomainDefAddImplicitInputDevice(virDomainDef *def)
 
 static int
 qemuDomainDefAddDefaultDevices(virDomainDefPtr def,
-                               virQEMUCapsPtr qemuCaps)
+                               virQEMUCapsPtr qemuCaps,
+                               unsigned int parseFlags)
 {
     bool addDefaultUSB = true;
     int usbModel = -1; /* "default for machinetype" */
@@ -2680,10 +2681,33 @@ qemuDomainDefAddDefaultDevices(virDomainDefPtr def,
 
     case VIR_ARCH_ARMV7L:
     case VIR_ARCH_AARCH64:
-        addDefaultUSB = false;
-        addDefaultMemballoon = false;
-        if (qemuDomainIsVirt(def))
+        if (qemuDomainIsVirt(def)) {
+            /* All mach-virt guests get a PCIe Root, if supported by
+             * the QEMU binary */
             addPCIeRoot = virQEMUCapsGet(qemuCaps, QEMU_CAPS_OBJECT_GPEX);
+        }
+
+        if (qemuDomainIsVirt(def) &&
+            parseFlags & VIR_DOMAIN_DEF_PARSE_ABI_UPDATE) {
+            /* In addition to PCIe Root, newly-defined mach-virt guests
+             * also get a couple more devices so that they're more similar
+             * to guests on other architectures, notably x86/q35:
+             *
+             *   1) a USB3 controller, if supported by the QEMU binary;
+             *   2) a virtio memory balloon, as per the defaults defined
+             *      above.
+             */
+            if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_DEVICE_QEMU_XHCI))
+                usbModel = VIR_DOMAIN_CONTROLLER_MODEL_USB_QEMU_XHCI;
+            else
+                addDefaultUSB = false;
+        } else {
+            /* Other ARM guests (and existing mach-virt guests, in order
+             * to preserve guest ABI compatibility) don't get a PCIe Root,
+             * a USB controller or a memory balloon */
+            addDefaultUSB = false;
+            addDefaultMemballoon = false;
+        }
         break;
 
     case VIR_ARCH_PPC64:
@@ -3187,7 +3211,7 @@ qemuDomainDefPostParse(virDomainDefPtr def,
             goto cleanup;
     }
 
-    if (qemuDomainDefAddDefaultDevices(def, qemuCaps) < 0)
+    if (qemuDomainDefAddDefaultDevices(def, qemuCaps, parseFlags) < 0)
         goto cleanup;
 
     if (qemuCanonicalizeMachine(def, qemuCaps) < 0)
-- 
2.14.3

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