x86, hvm: Allow Cirrus VGA BIOS to clear framebuffer with minimal PIO writes.

Signed-off-by: Ben Guthro <bguthro@virtualron.com>
Signed-off-by: Gary Grebus <ggrebus@virtualiron.com>

2 files changed, 31 insertions, 7 deletions

diff --git a/tools/firmware/vgabios/clext.c b/tools/firmware/vgabios/clext.c
index 1eb3c69734..97ae080b2c 100644
--- a/tools/firmware/vgabios/clext.c
+++ b/tools/firmware/vgabios/clext.c
@@ -1489,14 +1489,26 @@ cirrus_clear_vram_1:
   mov dx, #0x3ce
   out dx, ax
   push ax
-  mov cx, #0xa000
-  mov es, cx
-  xor di, di
+
+;; Windows Vista appears to be emulating this sequence as part of changing 
+;; screen resolution, but it generates 4096 writes per iteration.
+;; Instead, use a magic register sequence to write the whole bank.
+;;mov cx, #0xa000
+;;mov es, cx
+;;xor di, di
+;;mov ax, si
+;;mov cx, #8192
+;;cld
+;;rep
+;;    stosw
   mov ax, si
-  mov cx, #8192
-  cld
-  rep
-      stosw
+  shl ax, #8
+  mov al, #0xfe
+  out dx, ax	;; Low byte of value to be written to the bank
+  mov ax, si
+  mov al, #0xff  
+  out dx, ax    ;; High byte and trigger the write
+
   pop ax
   inc ah
   cmp ah, bl
diff --git a/tools/ioemu/hw/cirrus_vga.c b/tools/ioemu/hw/cirrus_vga.c
index 2fd1626362..8a637c57fd 100644
--- a/tools/ioemu/hw/cirrus_vga.c
+++ b/tools/ioemu/hw/cirrus_vga.c
@@ -294,6 +294,7 @@ void *shared_vram;
 
 static void cirrus_bitblt_reset(CirrusVGAState *s);
 static void cirrus_update_memory_access(CirrusVGAState *s);
+static void cirrus_vga_mem_writew(void *opaque, target_phys_addr_t addr, uint32_t val);
 
 /***************************************
  *
@@ -1497,6 +1498,17 @@ cirrus_hook_write_gr(CirrusVGAState * s, unsigned reg_index, int reg_value)
     case 0x31:			// BLT STATUS/START
 	cirrus_write_bitblt(s, reg_value);
 	break;
+
+	// Extension to allow BIOS to clear 16K VRAM bank in one operation
+    case 0xFE:
+	s->gr[reg_index] = reg_value;  // Lower byte of value to be written
+	break;
+    case 0xFF: {
+	target_phys_addr_t addr;
+	for (addr = 0xa0000; addr < 0xa4000; addr += 2)
+	    cirrus_vga_mem_writew(s, addr, (reg_value << 8) | s->gr[0xFE]);
+	}
+	break;
     default:
 #ifdef DEBUG_CIRRUS
 	printf("cirrus: outport gr_index %02x, gr_value %02x\n", reg_index,