[PATCH] bochs-display: add dummy mmio handler

[Gerd Hoffmann <kraxel@redhat.com>]

The bochs-display mmio bar has some sub-regions with the actual hardware
registers.  What happens when the guest access something outside those
regions depends on the archirecture.  On x86 those reads succeed (and
return 0xff I think).  On risc-v qemu aborts.

This patch adds handlers for the parent region, to make the wanted
behavior explicit and to make things consistent across architectures.

Cc: Alistair Francis <alistair23@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/display/bochs-display.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/hw/display/bochs-display.c b/hw/display/bochs-display.c
index 62085f9fc063..e93e838243b8 100644
--- a/hw/display/bochs-display.c
+++ b/hw/display/bochs-display.c
@@ -151,6 +151,26 @@ static const MemoryRegionOps bochs_display_qext_ops = {
     .endianness = DEVICE_LITTLE_ENDIAN,
 };
 
+static uint64_t dummy_read(void *ptr, hwaddr addr, unsigned size)
+{
+    return -1;
+}
+
+static void dummy_write(void *ptr, hwaddr addr,
+                        uint64_t val, unsigned size)
+{
+}
+
+static const MemoryRegionOps dummy_ops = {
+    .read = dummy_read,
+    .write = dummy_write,
+    .valid.min_access_size = 1,
+    .valid.max_access_size = 4,
+    .impl.min_access_size = 1,
+    .impl.max_access_size = 1,
+    .endianness = DEVICE_LITTLE_ENDIAN,
+};
+
 static int bochs_display_get_mode(BochsDisplayState *s,
                                    BochsDisplayMode *mode)
 {
@@ -284,8 +304,8 @@ static void bochs_display_realize(PCIDevice *dev, Error **errp)
     memory_region_init_io(&s->qext, obj, &bochs_display_qext_ops, s,
                           "qemu extended regs", PCI_VGA_QEXT_SIZE);
 
-    memory_region_init(&s->mmio, obj, "bochs-display-mmio",
-                       PCI_VGA_MMIO_SIZE);
+    memory_region_init_io(&s->mmio, obj, &dummy_ops, NULL,
+                          "bochs-display-mmio", PCI_VGA_MMIO_SIZE);
     memory_region_add_subregion(&s->mmio, PCI_VGA_BOCHS_OFFSET, &s->vbe);
     memory_region_add_subregion(&s->mmio, PCI_VGA_QEXT_OFFSET, &s->qext);
 
-- 
2.18.2