[PATCH RFC v1 1/1] hw/s390x/ccw: Disable legacy virtio-pci by default (v11.1+)

[Jaehoon Kim <jhkim@linux.ibm.com>]

On the s390 Linux kernel, IO_SPACE_LIMIT has been 0 since the initial
zPCI implementation (commit cd24834130ac "s390/pci: base support"),
making I/O BARs unusable.

However, when virtio-pci devices operate in transitional mode, QEMU
unconditionally exposes the legacy interface via BAR0. This results in
firmware warnings during PCI enumeration, such as:

  pci 0005:00:00.0: [Firmware Bug]: BAR 0: invalid; can't size

even though BAR0 is never usable on the s390 kernel.

Close this gap by disabling legacy virtio-pci support starting from
machine version 11.1. This effectively makes virtio-pci devices
non-transitional and prevents the creation of the unusable legacy I/O
BAR.

This introduces s390x-specific global compatibility properties that
set disable-legacy=on as the default for virtio-pci devices. Machine
versions v11.0 and earlier set disable-legacy=off to maintain their
original default behavior (legacy support enabled), ensuring VMs
created with those versions continue to work identically.

Users can override the default on the command line if needed:
  - On v11.1+: -global virtio-pci.disable-legacy=off (to enable legacy)
  - On v11.0-: -global virtio-pci.disable-legacy=on (to disable legacy)

Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
---
 hw/s390x/s390-virtio-ccw.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index cc768daeb0..6fbea1e289 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -788,6 +788,18 @@ static void machine_set_loadparm(Object *obj, Visitor *v,
     g_free(val);
 }
 
+/*
+ * S390x-specific global compatibility properties.
+ *
+ * Set disable-legacy=on as the default for virtio-pci devices starting
+ * from v11.1. Machine versions v11.0 and earlier set disable-legacy=off
+ * to maintain their original default behavior (legacy support enabled).
+ */
+static GlobalProperty hw_compat_s390x[] = {
+    { TYPE_VIRTIO_PCI, "disable-legacy", "on", .optional = true},
+};
+static const size_t hw_compat_s390x_len = G_N_ELEMENTS(hw_compat_s390x);
+
 static void ccw_machine_class_init(ObjectClass *oc, const void *data)
 {
     MachineClass *mc = MACHINE_CLASS(oc);
@@ -878,6 +890,9 @@ static const TypeInfo ccw_machine_info = {
         const void *data)                                                     \
     {                                                                         \
         MachineClass *mc = MACHINE_CLASS(oc);                                 \
+        /* Apply global s390x-wide default properties */                      \
+        compat_props_add(mc->compat_props, hw_compat_s390x,                   \
+                         hw_compat_s390x_len);                                \
         MACHINE_VER_SYM(class_options, ccw, __VA_ARGS__)(mc);                 \
         mc->desc = "Virtual s390x machine (version " MACHINE_VER_STR(__VA_ARGS__) ")"; \
         mc->init = MACHINE_VER_SYM(mach_init, ccw, __VA_ARGS__);              \
@@ -923,7 +938,15 @@ static void ccw_machine_11_0_instance_options(MachineState *machine)
 
 static void ccw_machine_11_0_class_options(MachineClass *mc)
 {
+    /*
+     * Preserve v11.0 and older version behavior:
+     * keep legacy virtio-pci enabled.
+     */
+    static GlobalProperty compat[] = {
+        { TYPE_VIRTIO_PCI, "disable-legacy", "off" },
+    };
     ccw_machine_11_1_class_options(mc);
+    compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
     compat_props_add(mc->compat_props, hw_compat_11_0, hw_compat_11_0_len);
 }
 DEFINE_CCW_MACHINE(11, 0);
-- 
2.50.1