KVM: Hack real-mode segments on vmx from KVM_SET_SREGS

As usual, we need to mangle segment registers when emulating real mode
as vm86 has specific constraints.  We special case the reset segment base,
and set the "access rights" (or descriptor flags) to vm86 comaptible values.

This fixes reboot on vmx.
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/vmx.c

@@ -864,7 +864,14 @@ static void vmx_set_segment(struct kvm_vcpu *vcpu,
 	vmcs_writel(sf->base, var->base);
 	vmcs_write32(sf->limit, var->limit);
 	vmcs_write16(sf->selector, var->selector);
-	if (var->unusable)
+	if (vcpu->rmode.active && var->s) {
+		/*
+		 * Hack real-mode segments into vm86 compatibility.
+		 */
+		if (var->base == 0xffff0000 && var->selector == 0xf000)
+			vmcs_writel(sf->base, 0xf0000);
+		ar = 0xf3;
+	} else if (var->unusable)
 		ar = 1 << 16;
 	else {
 		ar = var->type & 15;