KVM: X86: Optimize pte_list_desc with per-array counter

Add a counter field into pte_list_desc, so as to simplify the add/remove/loop
logic.  E.g., we don't need to loop over the array any more for most reasons.

This will make more sense after we've switched the array size to be larger
otherwise the counter will be a waste.

Initially I wanted to store a tail pointer at the head of the array list so we
don't need to traverse the list at least for pushing new ones (if without the
counter we traverse both the list and the array).  However that'll need
slightly more change without a huge lot benefit, e.g., after we grow entry
numbers per array the list traversing is not so expensive.

So let's be simple but still try to get as much benefit as we can with just
these extra few lines of changes (not to mention the code looks easier too
without looping over arrays).

I used the same a test case to fork 500 child and recycle them ("./rmap_fork
500" [1]), this patch further speeds up the total fork time of about 4%, which
is a total of 33% of vanilla kernel:

        Vanilla:      473.90 (+-5.93%)
        3->15 slots:  366.10 (+-4.94%)
        Add counter:  351.00 (+-3.70%)

[1] https://github.com/xzpeter/clibs/commit/825436f825453de2ea5aaee4bdb1c92281efe5b3Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210730220602.26327-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -138,11 +138,21 @@ module_param(dbg, bool, 0644);
 #include <trace/events/kvm.h>
 
 /* make pte_list_desc fit well in cache lines */
-#define PTE_LIST_EXT 15
+#define PTE_LIST_EXT 14
 
+/*
+ * Slight optimization of cacheline layout, by putting `more' and `spte_count'
+ * at the start; then accessing it will only use one single cacheline for
+ * either full (entries==PTE_LIST_EXT) case or entries<=6.
+ */
 struct pte_list_desc {
-	u64 *sptes[PTE_LIST_EXT];
 	struct pte_list_desc *more;
+	/*
+	 * Stores number of entries stored in the pte_list_desc.  No need to be
+	 * u64 but just for easier alignment.  When PTE_LIST_EXT, means full.
+	 */
+	u64 spte_count;
+	u64 *sptes[PTE_LIST_EXT];
 };
 
 struct kvm_shadow_walk_iterator {
@@ -901,7 +911,7 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
 			struct kvm_rmap_head *rmap_head)
 {
 	struct pte_list_desc *desc;
-	int i, count = 0;
+	int count = 0;
 
 	if (!rmap_head->val) {
 		rmap_printk("%p %llx 0->1\n", spte, *spte);
@@ -911,24 +921,24 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
 		desc = mmu_alloc_pte_list_desc(vcpu);
 		desc->sptes[0] = (u64 *)rmap_head->val;
 		desc->sptes[1] = spte;
+		desc->spte_count = 2;
 		rmap_head->val = (unsigned long)desc | 1;
 		++count;
 	} else {
 		rmap_printk("%p %llx many->many\n", spte, *spte);
 		desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
-		while (desc->sptes[PTE_LIST_EXT-1]) {
+		while (desc->spte_count == PTE_LIST_EXT) {
 			count += PTE_LIST_EXT;
-
 			if (!desc->more) {
 				desc->more = mmu_alloc_pte_list_desc(vcpu);
 				desc = desc->more;
+				desc->spte_count = 0;
 				break;
 			}
 			desc = desc->more;
 		}
-		for (i = 0; desc->sptes[i]; ++i)
-			++count;
-		desc->sptes[i] = spte;
+		count += desc->spte_count;
+		desc->sptes[desc->spte_count++] = spte;
 	}
 	return count;
 }
@@ -938,13 +948,12 @@ pte_list_desc_remove_entry(struct kvm_rmap_head *rmap_head,
 			   struct pte_list_desc *desc, int i,
 			   struct pte_list_desc *prev_desc)
 {
-	int j;
+	int j = desc->spte_count - 1;
 
-	for (j = PTE_LIST_EXT - 1; !desc->sptes[j] && j > i; --j)
-		;
 	desc->sptes[i] = desc->sptes[j];
 	desc->sptes[j] = NULL;
-	if (j != 0)
+	desc->spte_count--;
+	if (desc->spte_count)
 		return;
 	if (!prev_desc && !desc->more)
 		rmap_head->val = 0;
@@ -977,7 +986,7 @@ static void __pte_list_remove(u64 *spte, struct kvm_rmap_head *rmap_head)
 		desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
 		prev_desc = NULL;
 		while (desc) {
-			for (i = 0; i < PTE_LIST_EXT && desc->sptes[i]; ++i) {
+			for (i = 0; i < desc->spte_count; ++i) {
 				if (desc->sptes[i] == spte) {
 					pte_list_desc_remove_entry(rmap_head,
 							desc, i, prev_desc);