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>

1 files changed, 4 insertions, 1 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index cb2efa463793..4c5ff22d101a 100644
--- a/arch/x86/kvm/emulate.c
+++ b/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;