[PATCH 17/22] KVM: x86/mmu: Refactor low level rmap helpers to prep for walking w/o mmu_lock

[Sean Christopherson <seanjc@google.com>]

Refactor the pte_list and rmap code to always read and write rmap_head->val
exactly once, e.g. by collecting changes in a local variable and then
propagating those changes back to rmap_head->val as appropriate.  This will
allow implementing a per-rmap rwlock (of sorts) by adding a LOCKED bit into
the rmap value alongside the MANY bit.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 83 +++++++++++++++++++++++++-----------------
 1 file changed, 50 insertions(+), 33 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 73569979130d..49c1f6cc1526 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -920,21 +920,24 @@ static struct kvm_memory_slot *gfn_to_memslot_dirty_bitmap(struct kvm_vcpu *vcpu
 static int pte_list_add(struct kvm_mmu_memory_cache *cache, u64 *spte,
 			struct kvm_rmap_head *rmap_head)
 {
+	unsigned long old_val, new_val;
 	struct pte_list_desc *desc;
 	int count = 0;
 
-	if (!rmap_head->val) {
-		rmap_head->val = (unsigned long)spte;
-	} else if (!(rmap_head->val & KVM_RMAP_MANY)) {
+	old_val = rmap_head->val;
+
+	if (!old_val) {
+		new_val = (unsigned long)spte;
+	} else if (!(old_val & KVM_RMAP_MANY)) {
 		desc = kvm_mmu_memory_cache_alloc(cache);
-		desc->sptes[0] = (u64 *)rmap_head->val;
+		desc->sptes[0] = (u64 *)old_val;
 		desc->sptes[1] = spte;
 		desc->spte_count = 2;
 		desc->tail_count = 0;
-		rmap_head->val = (unsigned long)desc | KVM_RMAP_MANY;
+		new_val = (unsigned long)desc | KVM_RMAP_MANY;
 		++count;
 	} else {
-		desc = (struct pte_list_desc *)(rmap_head->val & ~KVM_RMAP_MANY);
+		desc = (struct pte_list_desc *)(old_val & ~KVM_RMAP_MANY);
 		count = desc->tail_count + desc->spte_count;
 
 		/*
@@ -943,21 +946,25 @@ static int pte_list_add(struct kvm_mmu_memory_cache *cache, u64 *spte,
 		 */
 		if (desc->spte_count == PTE_LIST_EXT) {
 			desc = kvm_mmu_memory_cache_alloc(cache);
-			desc->more = (struct pte_list_desc *)(rmap_head->val & ~KVM_RMAP_MANY);
+			desc->more = (struct pte_list_desc *)(old_val & ~KVM_RMAP_MANY);
 			desc->spte_count = 0;
 			desc->tail_count = count;
-			rmap_head->val = (unsigned long)desc | KVM_RMAP_MANY;
+			new_val = (unsigned long)desc | KVM_RMAP_MANY;
+		} else {
+			new_val = old_val;
 		}
 		desc->sptes[desc->spte_count++] = spte;
 	}
+
+	rmap_head->val = new_val;
+
 	return count;
 }
 
-static void pte_list_desc_remove_entry(struct kvm *kvm,
-				       struct kvm_rmap_head *rmap_head,
+static void pte_list_desc_remove_entry(struct kvm *kvm, unsigned long *rmap_val,
 				       struct pte_list_desc *desc, int i)
 {
-	struct pte_list_desc *head_desc = (struct pte_list_desc *)(rmap_head->val & ~KVM_RMAP_MANY);
+	struct pte_list_desc *head_desc = (struct pte_list_desc *)(*rmap_val & ~KVM_RMAP_MANY);
 	int j = head_desc->spte_count - 1;
 
 	/*
@@ -984,9 +991,9 @@ static void pte_list_desc_remove_entry(struct kvm *kvm,
 	 * head at the next descriptor, i.e. the new head.
 	 */
 	if (!head_desc->more)
-		rmap_head->val = 0;
+		*rmap_val = 0;
 	else
-		rmap_head->val = (unsigned long)head_desc->more | KVM_RMAP_MANY;
+		*rmap_val = (unsigned long)head_desc->more | KVM_RMAP_MANY;
 	mmu_free_pte_list_desc(head_desc);
 }
 
@@ -994,24 +1001,26 @@ static void pte_list_remove(struct kvm *kvm, u64 *spte,
 			    struct kvm_rmap_head *rmap_head)
 {
 	struct pte_list_desc *desc;
+	unsigned long rmap_val;
 	int i;
 
-	if (KVM_BUG_ON_DATA_CORRUPTION(!rmap_head->val, kvm))
-		return;
+	rmap_val = rmap_head->val;
+	if (KVM_BUG_ON_DATA_CORRUPTION(!rmap_val, kvm))
+		goto out;
 
-	if (!(rmap_head->val & KVM_RMAP_MANY)) {
-		if (KVM_BUG_ON_DATA_CORRUPTION((u64 *)rmap_head->val != spte, kvm))
-			return;
+	if (!(rmap_val & KVM_RMAP_MANY)) {
+		if (KVM_BUG_ON_DATA_CORRUPTION((u64 *)rmap_val != spte, kvm))
+			goto out;
 
-		rmap_head->val = 0;
+		rmap_val = 0;
 	} else {
-		desc = (struct pte_list_desc *)(rmap_head->val & ~KVM_RMAP_MANY);
+		desc = (struct pte_list_desc *)(rmap_val & ~KVM_RMAP_MANY);
 		while (desc) {
 			for (i = 0; i < desc->spte_count; ++i) {
 				if (desc->sptes[i] == spte) {
-					pte_list_desc_remove_entry(kvm, rmap_head,
+					pte_list_desc_remove_entry(kvm, &rmap_val,
 								   desc, i);
-					return;
+					goto out;
 				}
 			}
 			desc = desc->more;
@@ -1019,6 +1028,9 @@ static void pte_list_remove(struct kvm *kvm, u64 *spte,
 
 		KVM_BUG_ON_DATA_CORRUPTION(true, kvm);
 	}
+
+out:
+	rmap_head->val = rmap_val;
 }
 
 static void kvm_zap_one_rmap_spte(struct kvm *kvm,
@@ -1033,17 +1045,19 @@ static bool kvm_zap_all_rmap_sptes(struct kvm *kvm,
 				   struct kvm_rmap_head *rmap_head)
 {
 	struct pte_list_desc *desc, *next;
+	unsigned long rmap_val;
 	int i;
 
-	if (!rmap_head->val)
+	rmap_val = rmap_head->val;
+	if (!rmap_val)
 		return false;
 
-	if (!(rmap_head->val & KVM_RMAP_MANY)) {
-		mmu_spte_clear_track_bits(kvm, (u64 *)rmap_head->val);
+	if (!(rmap_val & KVM_RMAP_MANY)) {
+		mmu_spte_clear_track_bits(kvm, (u64 *)rmap_val);
 		goto out;
 	}
 
-	desc = (struct pte_list_desc *)(rmap_head->val & ~KVM_RMAP_MANY);
+	desc = (struct pte_list_desc *)(rmap_val & ~KVM_RMAP_MANY);
 
 	for (; desc; desc = next) {
 		for (i = 0; i < desc->spte_count; i++)
@@ -1059,14 +1073,15 @@ static bool kvm_zap_all_rmap_sptes(struct kvm *kvm,
 
 unsigned int pte_list_count(struct kvm_rmap_head *rmap_head)
 {
+	unsigned long rmap_val = rmap_head->val;
 	struct pte_list_desc *desc;
 
-	if (!rmap_head->val)
+	if (!rmap_val)
 		return 0;
-	else if (!(rmap_head->val & KVM_RMAP_MANY))
+	else if (!(rmap_val & KVM_RMAP_MANY))
 		return 1;
 
-	desc = (struct pte_list_desc *)(rmap_head->val & ~KVM_RMAP_MANY);
+	desc = (struct pte_list_desc *)(rmap_val & ~KVM_RMAP_MANY);
 	return desc->tail_count + desc->spte_count;
 }
 
@@ -1109,6 +1124,7 @@ static void rmap_remove(struct kvm *kvm, u64 *spte)
  */
 struct rmap_iterator {
 	/* private fields */
+	struct rmap_head *head;
 	struct pte_list_desc *desc;	/* holds the sptep if not NULL */
 	int pos;			/* index of the sptep */
 };
@@ -1123,18 +1139,19 @@ struct rmap_iterator {
 static u64 *rmap_get_first(struct kvm_rmap_head *rmap_head,
 			   struct rmap_iterator *iter)
 {
+	unsigned long rmap_val = rmap_head->val;
 	u64 *sptep;
 
-	if (!rmap_head->val)
+	if (!rmap_val)
 		return NULL;
 
-	if (!(rmap_head->val & KVM_RMAP_MANY)) {
+	if (!(rmap_val & KVM_RMAP_MANY)) {
 		iter->desc = NULL;
-		sptep = (u64 *)rmap_head->val;
+		sptep = (u64 *)rmap_val;
 		goto out;
 	}
 
-	iter->desc = (struct pte_list_desc *)(rmap_head->val & ~KVM_RMAP_MANY);
+	iter->desc = (struct pte_list_desc *)(rmap_val & ~KVM_RMAP_MANY);
 	iter->pos = 0;
 	sptep = iter->desc->sptes[iter->pos];
 out:
-- 
2.46.0.76.ge559c4bf1a-goog