[PATCH 2/4] KVM: MMU: simplify the logic of __mmu_unsync_walk()

[Xiao Guangrong]

Each time i looked into the logic of walking unsync shadow pages, it costs
lots of time to understand what it is doing. The trick of this logic is
that the item, sp and idx, saved to kvm_mmu_pages is the sp and the index
in the _parent_ level and it lacks any comment to explain this fact

This patch simplifies it by saving the sp and its index to kvm_mmu_pages,
then it is much easier to understand the operations on the its index

Signed-off-by: Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxxx>
---
 arch/x86/kvm/mmu.c | 40 ++++++++++++++++++++++------------------
 1 file changed, 22 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 6bdfbc2..e273144 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1830,6 +1830,8 @@ static inline void clear_unsync_child_bit(struct kvm_mmu_page *sp, int idx)
 	__clear_bit(idx, sp->unsync_child_bitmap);
 }
 
+#define INVALID_INDEX (-1)
+
 static int __mmu_unsync_walk(struct kvm_mmu_page *sp,
 			   struct kvm_mmu_pages *pvec)
 {
@@ -1846,10 +1848,10 @@ static int __mmu_unsync_walk(struct kvm_mmu_page *sp,
 
 		child = page_header(ent & PT64_BASE_ADDR_MASK);
 
-		if (child->unsync_children) {
-			if (mmu_pages_add(pvec, child, i))
-				return -ENOSPC;
+		if (mmu_pages_add(pvec, sp, i))
+			return -ENOSPC;
 
+		if (child->unsync_children) {
 			ret = __mmu_unsync_walk(child, pvec);
 			if (!ret) {
 				clear_unsync_child_bit(sp, i);
@@ -1860,7 +1862,13 @@ static int __mmu_unsync_walk(struct kvm_mmu_page *sp,
 				return ret;
 		} else if (child->unsync) {
 			nr_unsync_leaf++;
-			if (mmu_pages_add(pvec, child, i))
+
+			/*
+			 * the unsync is on the last level so its 'idx' is
+			 * useless, we set it to INVALID_INDEX to catch
+			 * potential bugs.
+			 */
+			if (mmu_pages_add(pvec, child, INVALID_INDEX))
 				return -ENOSPC;
 		} else
 			clear_unsync_child_bit(sp, i);
@@ -1869,8 +1877,6 @@ static int __mmu_unsync_walk(struct kvm_mmu_page *sp,
 	return nr_unsync_leaf;
 }
 
-#define INVALID_INDEX (-1)
-
 static int mmu_unsync_walk(struct kvm_mmu_page *sp,
 			   struct kvm_mmu_pages *pvec)
 {
@@ -1878,7 +1884,6 @@ static int mmu_unsync_walk(struct kvm_mmu_page *sp,
 	if (!sp->unsync_children)
 		return 0;
 
-	mmu_pages_add(pvec, sp, INVALID_INDEX);
 	return __mmu_unsync_walk(sp, pvec);
 }
 
@@ -1994,16 +1999,18 @@ static int mmu_pages_next(struct kvm_mmu_pages *pvec,
 {
 	int n;
 
-	for (n = i+1; n < pvec->nr; n++) {
+	for (n = i + 1; n < pvec->nr; n++) {
 		struct kvm_mmu_page *sp = pvec->page[n].sp;
 		unsigned idx = pvec->page[n].idx;
 		int level = sp->role.level;
 
-		parents->idx[level-1] = idx;
-		if (level == PT_PAGE_TABLE_LEVEL)
+		if (level == PT_PAGE_TABLE_LEVEL) {
+			WARN_ON(idx != INVALID_INDEX);
 			break;
+		}
 
-		parents->parent[level-2] = sp;
+		parents->idx[level - 2] = idx;
+		parents->parent[level - 2] = sp;
 	}
 
 	return n;
@@ -2018,19 +2025,16 @@ static int mmu_pages_first(struct kvm_mmu_pages *pvec,
 	if (pvec->nr == 0)
 		return 0;
 
-	WARN_ON(pvec->page[0].idx != INVALID_INDEX);
-
 	sp = pvec->page[0].sp;
 	level = sp->role.level;
 	WARN_ON(level == PT_PAGE_TABLE_LEVEL);
 
-	parents->parent[level-2] = sp;
-
-	/* Also set up a sentinel.  Further entries in pvec are all
+	/*
+	 * Also set up a sentinel. Further entries in pvec are all
 	 * children of sp, so this element is never overwritten.
 	 */
-	parents->parent[level-1] = NULL;
-	return mmu_pages_next(pvec, parents, 0);
+	parents->parent[level - 1] = NULL;
+	return mmu_pages_next(pvec, parents, -1);
 }
 
 static void mmu_pages_clear_parents(struct mmu_page_path *parents)
-- 
1.8.3.1