KVM: Introduce gfn_to_index() which returns the index for a given level

This patch cleans up the code and removes the "(void)level;" warning
suppressor.

Note that we can also use this for PT_PAGE_TABLE_LEVEL to treat every
level uniformly later.
Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/mmu.c

@@ -688,8 +688,7 @@ static struct kvm_lpage_info *lpage_info_slot(gfn_t gfn,
 {
 	unsigned long idx;
 
-	idx = (gfn >> KVM_HPAGE_GFN_SHIFT(level)) -
-	      (slot->base_gfn >> KVM_HPAGE_GFN_SHIFT(level));
+	idx = gfn_to_index(gfn, slot->base_gfn, level);
 	return &slot->lpage_info[level - 2][idx];
 }
 

include/linux/kvm_host.h

@@ -681,6 +681,13 @@ static inline int memslot_id(struct kvm *kvm, gfn_t gfn)
 	return gfn_to_memslot(kvm, gfn)->id;
 }
 
+static inline gfn_t gfn_to_index(gfn_t gfn, gfn_t base_gfn, int level)
+{
+	/* KVM_HPAGE_GFN_SHIFT(PT_PAGE_TABLE_LEVEL) must be 0. */
+	return (gfn >> KVM_HPAGE_GFN_SHIFT(level)) -
+		(base_gfn >> KVM_HPAGE_GFN_SHIFT(level));
+}
+
 static inline unsigned long gfn_to_hva_memslot(struct kvm_memory_slot *slot,
 					       gfn_t gfn)
 {

virt/kvm/kvm_main.c

@@ -784,15 +784,10 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		int lpages;
 		int level = i + 2;
 
-		/* Avoid unused variable warning if no large pages */
-		(void)level;
-
 		if (new.lpage_info[i])
 			continue;
 
-		lpages = 1 + ((base_gfn + npages - 1)
-			     >> KVM_HPAGE_GFN_SHIFT(level));
-		lpages -= base_gfn >> KVM_HPAGE_GFN_SHIFT(level);
+		lpages = gfn_to_index(base_gfn + npages - 1, base_gfn, level) + 1;
 
 		new.lpage_info[i] = vzalloc(lpages * sizeof(*new.lpage_info[i]));