KVM: x86 emulator: Handle wraparound in (cs_base + offset) when fetching insns

Currently, setting a large (i.e. negative) base address for %cs does not work on
a 64-bit host. The "JOS" teaching operating system, used by MIT and other
universities, relies on such segments while bootstrapping its way to full
virtual memory management.
Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/emulate.c

@@ -637,9 +637,12 @@ static int do_fetch_insn_byte(struct x86_emulate_ctxt *ctxt,
 	int size, cur_size;
 
 	if (eip == fc->end) {
+		unsigned long linear = eip + ctxt->cs_base;
+		if (ctxt->mode != X86EMUL_MODE_PROT64)
+			linear &= (u32)-1;
 		cur_size = fc->end - fc->start;
 		size = min(15UL - cur_size, PAGE_SIZE - offset_in_page(eip));
-		rc = ops->fetch(ctxt->cs_base + eip, fc->data + cur_size,
+		rc = ops->fetch(linear, fc->data + cur_size,
 				size, ctxt->vcpu, &ctxt->exception);
 		if (rc != X86EMUL_CONTINUE)
 			return rc;