[PATCH v4] piix: fix xenfv regression, add compat machine xenfv-qemu4

[Olaf Hering <olaf@aepfle.de>]

With qemu4 an incompatible change was added to pc_piix, which makes it
practical impossible to migrate domUs started with qemu2 or qemu3 to
newer qemu versions. Commit 7fccf2a06890e3bc3b30e29827ad3fb93fe88fea
added and enabled a new member "smbus_no_migration_support". In commit
4ab2f2a8aabfea95cc53c64e13b3f67960b27fdf the vmstate_acpi got new
elements, which are conditionally filled. As a result, an incoming
migration expected smbus related data unless smbus migration was
disabled for a given MachineClass. Since first commit forgot to handle
'xenfv', domUs started with qemu4 are incompatible with their qemu3
siblings.

Using other existing machine types, such as 'pc-i440fx-3.1', is not
possible because 'xenfv' creates the 'xen-platform' PCI device at
00:02.0, while all other variants to run a domU would create it at
00:04.0.

To cover both the existing and the broken case of 'xenfv' in a single
qemu binary, a new compatibility variant of 'xenfv-qemu4' must be added
which targets domUs started with qemu-4.0, qemu-4.1 and qemu-4.2. The
existing 'xenfv' restores compatibility of qemu5+ with qemu2/3.

Host admins who started domUs with qemu-4.x have to use a wrapper script
which appends '-machine xenfv-qemu4' to the device-model command line.
This is only required if there is no maintenance window which allows to
temporary shutdown the domU and restart it with a fixed device-model.

The wrapper script is as simple as this:
  #!/bin/sh
  exec /usr/bin/qemu-system-i386 "$@" -machine xenfv-qemu4

With xl this script will be enabled with device_model_override=, see
xl.cfg(5). To live migrate a domU, adjust the existing domU.cfg and pass
it to xl migrate or xl save/restore:
  xl migrate -C new-domU.cfg domU remote-host
  xl save domU CheckpointFile new-domU.cfg
  xl restore new-domU.cfg CheckpointFile

With libvirt this script will be enabled with the <emulator> element in
domU.xml. Use 'virsh edit' prior 'virsh migrate' to replace the existing
<emulator> element to point it to the wrapper script.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
---
 hw/i386/pc_piix.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index e6756216f9..c565f41791 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -948,8 +948,21 @@ DEFINE_PC_MACHINE(isapc, "isapc", pc_init_isa,
 
 
 #ifdef CONFIG_XEN
+static void xenfv_qemu_4_x_machine_options(MachineClass *m)
+{
+    pc_i440fx_4_2_machine_options(m);
+    m->desc = "Xen Fully-virtualized PC (qemu 4.x compat)";
+    m->max_cpus = HVM_MAX_VCPUS;
+    m->default_machine_opts = "accel=xen";
+    m->smbus_no_migration_support = false;
+}
+
+DEFINE_PC_MACHINE(xenfv_qemu4, "xenfv-qemu4", pc_xen_hvm_init,
+                  xenfv_qemu_4_x_machine_options);
+
 static void xenfv_machine_options(MachineClass *m)
 {
+    pc_i440fx_3_1_machine_options(m);
     m->desc = "Xen Fully-virtualized PC";
     m->max_cpus = HVM_MAX_VCPUS;
     m->default_machine_opts = "accel=xen";