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>
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index cb2efa4..4c5ff22 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -637,9 +637,12 @@
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;