vgabios: Don't use extra stack if it appears a modern OS is in use

If the last mode set (while not in vm86 mode) was done from a VBE mode
set call then disable the extra stack.  This works under the premise
that only a modern OS would invoke the VBE mode changing facilities
and a modern OS would always call the vgabios with sufficient stack
space.

This is an ugly hack to work around a problem Windows Vista (and
possibly later Windows releases) has with the VGA BIOS using a stack
in the e-segment.

Reported-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/x86.h b/src/x86.h
index 7798b1c1..14ebb7de 100644
--- a/src/x86.h
+++ b/src/x86.h
@@ -83,6 +83,11 @@ static inline u32 getcr0(void) {
 static inline void setcr0(u32 cr0) {
     asm("movl %0, %%cr0" : : "r"(cr0));
 }
+static inline u16 getcr0_vm86(void) {
+    u16 cr0;
+    asm("smsww %0" : "=r"(cr0));
+    return cr0;
+}
 
 static inline u64 rdmsr(u32 index)
 {