KVM: x86: NOP emulation clears (incorrectly) the high 32-bits of RAX

On long-mode the current NOP (0x90) emulation still writes back to RAX.  As a
result, EAX is zero-extended and the high 32-bits of RAX are cleared.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 6f09b2e555ef..84dc4ba0364d 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4688,8 +4688,9 @@ special_insn:
                 break;
         case 0x90 ... 0x97: /* nop / xchg reg, rax */
                 if (ctxt->dst.addr.reg == reg_rmw(ctxt, VCPU_REGS_RAX))
-                        break;
-                rc = em_xchg(ctxt);
+                        ctxt->dst.type = OP_NONE;
+                else
+                        rc = em_xchg(ctxt);
                 break;
         case 0x98: /* cbw/cwde/cdqe */
                 switch (ctxt->op_bytes) {