KVM: Workaround vmx inability to virtualize the reset state

The reset state has cs.selector == 0xf000 and cs.base == 0xffff0000,
which aren't compatible with vm86 mode, which is used for real mode
virtualization.

When we create a vcpu, we set cs.base to 0xf0000, but if we get there by
way of a reset, the values are inconsistent and vmx refuses to enter
guest mode.

Workaround by detecting the state and munging it appropriately.
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/vmx.c

@@ -712,6 +712,8 @@ static void enter_rmode(struct kvm_vcpu *vcpu)
 
 	vmcs_write32(GUEST_CS_AR_BYTES, 0xf3);
 	vmcs_write32(GUEST_CS_LIMIT, 0xffff);
+	if (vmcs_readl(GUEST_CS_BASE) == 0xffff0000)
+		vmcs_writel(GUEST_CS_BASE, 0xf0000);
 	vmcs_write16(GUEST_CS_SELECTOR, vmcs_readl(GUEST_CS_BASE) >> 4);
 
 	fix_rmode_seg(VCPU_SREG_ES, &vcpu->rmode.es);