Quoting cirrus source code:
   Follow real hardware, cirrus card emulated has 4 MB video memory.
   Also accept 8 MB/16 MB for backward compatibility.

So just use 4MB by default.  We decided to leave that at 8MB by default
a while ago, for live migration compatibility reasons.  But we have
compat properties to handle that, so that isn't a compeling reason.

This also removes some sanity check inconsistencies in the cirrus code.
Some places check against the allocated video memory, some places check
against the 4MB physical hardware has.  Guest code can trigger asserts
because of that.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/display/cirrus_vga.c | 4 ++--
 include/hw/compat.h     | 8 ++++++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c
index b9e7cb1..efbd2c1 100644
--- a/hw/display/cirrus_vga.c
+++ b/hw/display/cirrus_vga.c
@@ -3029,7 +3029,7 @@ static void isa_cirrus_vga_realizefn(DeviceState *dev, Error **errp)
 
 static Property isa_cirrus_vga_properties[] = {
     DEFINE_PROP_UINT32("vgamem_mb", struct ISACirrusVGAState,
-                       cirrus_vga.vga.vram_size_mb, 8),
+                       cirrus_vga.vga.vram_size_mb, 4),
     DEFINE_PROP_END_OF_LIST(),
 };
 
@@ -3098,7 +3098,7 @@ static void pci_cirrus_vga_realize(PCIDevice *dev, Error **errp)
 
 static Property pci_vga_cirrus_properties[] = {
     DEFINE_PROP_UINT32("vgamem_mb", struct PCICirrusVGAState,
-                       cirrus_vga.vga.vram_size_mb, 8),
+                       cirrus_vga.vga.vram_size_mb, 4),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/include/hw/compat.h b/include/hw/compat.h
index b7db438..b7e0e9f 100644
--- a/include/hw/compat.h
+++ b/include/hw/compat.h
@@ -18,6 +18,14 @@
         .driver   = "pci-bridge",\
         .property = "shpc",\
         .value    = "on",\
+    },{\
+        .driver   = "cirrus-vga",\
+        .property = "vgamem_mb",\
+        .value    = "8",\
+    },{\
+        .driver   = "isa-cirrus-vga",\
+        .property = "vgamem_mb",\
+        .value    = "8",\
     },
 
 #define HW_COMPAT_2_7 \
-- 
1.8.3.1