kvm: disable uninitialized var warning

I see this in 3.5-rc1:

arch/x86/kvm/mmu.c: In function ‘kvm_test_age_rmapp’:
arch/x86/kvm/mmu.c:1271: warning: ‘iter.desc’ may be used uninitialized in this function

The line in question was introduced by commit 1e3f42f03c38c29c1814199a6f0a2f01b919ea3f

static int kvm_test_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
unsigned long data)
{

-       u64 *spte;
+       u64 *sptep;
+       struct rmap_iterator iter;   <- line 1271
        int young = 0;
 
        /*

The reason I think is that the compiler assumes that the rmap value could be 0, so

static u64 *rmap_get_first(unsigned long rmap, struct rmap_iterator *iter)

{
        if (!rmap)
                return NULL;

        if (!(rmap & 1)) {
                iter->desc = NULL;
                return (u64 *)rmap;
        }

        iter->desc = (struct pte_list_desc *)(rmap & ~1ul);
        iter->pos = 0;
        return iter->desc->sptes[iter->pos];
}

will not initialize iter.desc, but the compiler isn't smart enough to see that

        for (sptep = rmap_get_first(*rmapp, &iter); sptep;
             sptep = rmap_get_next(&iter)) {

will immediately exit in this case.
I checked by adding

        if (!*rmapp)
                goto out;

on top which is clearly equivalent but disables the warning.

This patch uses uninitialized_var to disable the warning without increasing code size.

---

index be3cea4..dc83761 100644
@@ -1238,7 +1238,7 @@ static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
 			 unsigned long data)
 {
 	u64 *sptep;
-	struct rmap_iterator iter;
+	struct rmap_iterator uninitialized_var(iter);
 	int young = 0;
 
 	/*

MST

Thanks, applied.

error compiling committee.c: too many arguments to function