vga: fix vram double-mapping with -vga std and -M pc-0.12

With pc-0.12, we map the video RAM both through the PCI BAR (the guest does
this) and through a fixed mapping at 0xe0000000.  The memory API doesn't allow
this double map, and aborts.

Fix by using an alias.

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/vga.c

@@ -2357,10 +2357,15 @@ void vga_init(VGACommonState *s, MemoryRegion *address_space,
 void vga_init_vbe(VGACommonState *s, MemoryRegion *system_memory)
 {
 #ifdef CONFIG_BOCHS_VBE
+    /* With pc-0.12 and below we map both the PCI BAR and the fixed VBE region,
+     * so use an alias to avoid double-mapping the same region.
+     */
+    memory_region_init_alias(&s->vram_vbe, "vram.vbe",
+                             &s->vram, 0, memory_region_size(&s->vram));
     /* XXX: use optimized standard vga accesses */
     memory_region_add_subregion(system_memory,
                                 VBE_DISPI_LFB_PHYSICAL_ADDRESS,
-                                &s->vram);
+                                &s->vram_vbe);
     s->vbe_mapped = 1;
 #endif 
 }

hw/vga_int.h

@@ -105,6 +105,7 @@ typedef struct VGACommonState {
     MemoryRegion *legacy_address_space;
     uint8_t *vram_ptr;
     MemoryRegion vram;
+    MemoryRegion vram_vbe;
     uint32_t vram_size;
     uint32_t latch;
     MemoryRegion *chain4_alias;