KVM: VMX: Fix kvm_set_shared_msr() called in preemptible context

kvm_set_shared_msr() may not be called in preemptible context,
but vmx_set_msr() does so:

  BUG: using smp_processor_id() in preemptible [00000000] code: qemu-kvm/22713
  caller is kvm_set_shared_msr+0x32/0xa0 [kvm]
  Pid: 22713, comm: qemu-kvm Not tainted 3.4.0-rc3+ #39
  Call Trace:
   [<ffffffff8131fa82>] debug_smp_processor_id+0xe2/0x100
   [<ffffffffa0328ae2>] kvm_set_shared_msr+0x32/0xa0 [kvm]
   [<ffffffffa03a103b>] vmx_set_msr+0x28b/0x2d0 [kvm_intel]
   ...

Making kvm_set_shared_msr() work in preemptible is cleaner, but
it's used in the fast path.  Making two variants is overkill, so
this patch just disables preemption around the call.
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/vmx.c

@@ -2210,9 +2210,12 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 data)
 		msr = find_msr_entry(vmx, msr_index);
 		if (msr) {
 			msr->data = data;
-			if (msr - vmx->guest_msrs < vmx->save_nmsrs)
+			if (msr - vmx->guest_msrs < vmx->save_nmsrs) {
+				preempt_disable();
 				kvm_set_shared_msr(msr->index, msr->data,
 						   msr->mask);
+				preempt_enable();
+			}
 			break;
 		}
 		ret = kvm_set_msr_common(vcpu, msr_index, data);