KVM: MMU: Improve iteration through sptes from rmap

Iteration using rmap_next(), the actual body is pte_list_next(), is
inefficient: every time we call it we start from checking whether rmap
holds a single spte or points to a descriptor which links more sptes.

In the case of shadow paging, this quadratic total iteration cost is a
problem.  Even for two dimensional paging, with EPT/NPT on, in which we
almost always have a single mapping, the extra checks at the end of the
iteration should be eliminated.

This patch fixes this by introducing rmap_iterator which keeps the
iteration context for the next search.  Furthermore the implementation
of rmap_next() is splitted into two functions, rmap_get_first() and
rmap_get_next(), to avoid repeatedly checking whether the rmap being
iterated on has only one spte.

Although there seemed to be only a slight change for EPT/NPT, the actual
improvement was significant: we observed that GET_DIRTY_LOG for 1GB
dirty memory became 15% faster than before.  This is probably because
the new code is easy to make branch predictions.

Note: we just remove pte_list_next() because we can think of parent_ptes
as a reverse mapping.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>

2 files changed, 124 insertions, 82 deletions

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 3213348e3a93..29ad6f9c58a5 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -842,32 +842,6 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
         return count;
 }
 
-static u64 *pte_list_next(unsigned long *pte_list, u64 *spte)
-{
-        struct pte_list_desc *desc;
-        u64 *prev_spte;
-        int i;
-
-        if (!*pte_list)
-                return NULL;
-        else if (!(*pte_list & 1)) {
-                if (!spte)
-                        return (u64 *)*pte_list;
-                return NULL;
-        }
-        desc = (struct pte_list_desc *)(*pte_list & ~1ul);
-        prev_spte = NULL;
-        while (desc) {
-                for (i = 0; i < PTE_LIST_EXT && desc->sptes[i]; ++i) {
-                        if (prev_spte == spte)
-                                return desc->sptes[i];
-                        prev_spte = desc->sptes[i];
-                }
-                desc = desc->more;
-        }
-        return NULL;
-}
-
 static void
 pte_list_desc_remove_entry(unsigned long *pte_list, struct pte_list_desc *desc,
                            int i, struct pte_list_desc *prev_desc)
@@ -988,11 +962,6 @@ static int rmap_add(struct kvm_vcpu *vcpu, u64 *spte, gfn_t gfn)
         return pte_list_add(vcpu, spte, rmapp);
 }
 
-static u64 *rmap_next(unsigned long *rmapp, u64 *spte)
-{
-        return pte_list_next(rmapp, spte);
-}
-
 static void rmap_remove(struct kvm *kvm, u64 *spte)
 {
         struct kvm_mmu_page *sp;
@@ -1005,6 +974,67 @@ static void rmap_remove(struct kvm *kvm, u64 *spte)
         pte_list_remove(spte, rmapp);
 }
 
+/*
+ * Used by the following functions to iterate through the sptes linked by a
+ * rmap.  All fields are private and not assumed to be used outside.
+ */
+struct rmap_iterator {
+        /* private fields */
+        struct pte_list_desc *desc;     /* holds the sptep if not NULL */
+        int pos;                        /* index of the sptep */
+};
+
+/*
+ * Iteration must be started by this function.  This should also be used after
+ * removing/dropping sptes from the rmap link because in such cases the
+ * information in the itererator may not be valid.
+ *
+ * Returns sptep if found, NULL otherwise.
+ */
+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];
+}
+
+/*
+ * Must be used with a valid iterator: e.g. after rmap_get_first().
+ *
+ * Returns sptep if found, NULL otherwise.
+ */
+static u64 *rmap_get_next(struct rmap_iterator *iter)
+{
+        if (iter->desc) {
+                if (iter->pos < PTE_LIST_EXT - 1) {
+                        u64 *sptep;
+
+                        ++iter->pos;
+                        sptep = iter->desc->sptes[iter->pos];
+                        if (sptep)
+                                return sptep;
+                }
+
+                iter->desc = iter->desc->more;
+
+                if (iter->desc) {
+                        iter->pos = 0;
+                        /* desc->sptes[0] cannot be NULL */
+                        return iter->desc->sptes[iter->pos];
+                }
+        }
+
+        return NULL;
+}
+
 static void drop_spte(struct kvm *kvm, u64 *sptep)
 {
         if (mmu_spte_clear_track_bits(sptep))
@@ -1013,23 +1043,27 @@ static void drop_spte(struct kvm *kvm, u64 *sptep)
 
 static int __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
 {
-        u64 *spte = NULL;
+        u64 *sptep;
+        struct rmap_iterator iter;
         int write_protected = 0;
 
-        while ((spte = rmap_next(rmapp, spte))) {
-                BUG_ON(!(*spte & PT_PRESENT_MASK));
-                rmap_printk("rmap_write_protect: spte %p %llx\n", spte, *spte);
+        for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
+                BUG_ON(!(*sptep & PT_PRESENT_MASK));
+                rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
 
-                if (!is_writable_pte(*spte))
+                if (!is_writable_pte(*sptep)) {
+                        sptep = rmap_get_next(&iter);
                         continue;
+                }
 
                 if (level == PT_PAGE_TABLE_LEVEL) {
-                        mmu_spte_update(spte, *spte & ~PT_WRITABLE_MASK);
+                        mmu_spte_update(sptep, *sptep & ~PT_WRITABLE_MASK);
+                        sptep = rmap_get_next(&iter);
                 } else {
-                        BUG_ON(!is_large_pte(*spte));
-                        drop_spte(kvm, spte);
+                        BUG_ON(!is_large_pte(*sptep));
+                        drop_spte(kvm, sptep);
                         --kvm->stat.lpages;
-                        spte = NULL;
+                        sptep = rmap_get_first(*rmapp, &iter);
                 }
 
                 write_protected = 1;
@@ -1084,48 +1118,57 @@ static int rmap_write_protect(struct kvm *kvm, u64 gfn)
 static int kvm_unmap_rmapp(struct kvm *kvm, unsigned long *rmapp,
                            unsigned long data)
 {
-        u64 *spte;
+        u64 *sptep;
+        struct rmap_iterator iter;
         int need_tlb_flush = 0;
 
-        while ((spte = rmap_next(rmapp, NULL))) {
-                BUG_ON(!(*spte & PT_PRESENT_MASK));
-                rmap_printk("kvm_rmap_unmap_hva: spte %p %llx\n", spte, *spte);
-                drop_spte(kvm, spte);
+        while ((sptep = rmap_get_first(*rmapp, &iter))) {
+                BUG_ON(!(*sptep & PT_PRESENT_MASK));
+                rmap_printk("kvm_rmap_unmap_hva: spte %p %llx\n", sptep, *sptep);
+
+                drop_spte(kvm, sptep);
                 need_tlb_flush = 1;
         }
+
         return need_tlb_flush;
 }
 
 static int kvm_set_pte_rmapp(struct kvm *kvm, unsigned long *rmapp,
                              unsigned long data)
 {
+        u64 *sptep;
+        struct rmap_iterator iter;
         int need_flush = 0;
-        u64 *spte, new_spte;
+        u64 new_spte;
         pte_t *ptep = (pte_t *)data;
         pfn_t new_pfn;
 
         WARN_ON(pte_huge(*ptep));
         new_pfn = pte_pfn(*ptep);
-        spte = rmap_next(rmapp, NULL);
-        while (spte) {
-                BUG_ON(!is_shadow_present_pte(*spte));
-                rmap_printk("kvm_set_pte_rmapp: spte %p %llx\n", spte, *spte);
+
+        for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
+                BUG_ON(!is_shadow_present_pte(*sptep));
+                rmap_printk("kvm_set_pte_rmapp: spte %p %llx\n", sptep, *sptep);
+
                 need_flush = 1;
+
                 if (pte_write(*ptep)) {
-                        drop_spte(kvm, spte);
-                        spte = rmap_next(rmapp, NULL);
+                        drop_spte(kvm, sptep);
+                        sptep = rmap_get_first(*rmapp, &iter);
                 } else {
-                        new_spte = *spte &~ (PT64_BASE_ADDR_MASK);
+                        new_spte = *sptep & ~PT64_BASE_ADDR_MASK;
                         new_spte |= (u64)new_pfn << PAGE_SHIFT;
 
                         new_spte &= ~PT_WRITABLE_MASK;
                         new_spte &= ~SPTE_HOST_WRITEABLE;
                         new_spte &= ~shadow_accessed_mask;
-                        mmu_spte_clear_track_bits(spte);
-                        mmu_spte_set(spte, new_spte);
-                        spte = rmap_next(rmapp, spte);
+
+                        mmu_spte_clear_track_bits(sptep);
+                        mmu_spte_set(sptep, new_spte);
+                        sptep = rmap_get_next(&iter);
                 }
         }
+
         if (need_flush)
                 kvm_flush_remote_tlbs(kvm);
 
@@ -1184,7 +1227,8 @@ void kvm_set_spte_hva(struct kvm *kvm, unsigned long hva, pte_t pte)
 static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
                          unsigned long data)
 {
-        u64 *spte;
+        u64 *sptep;
+        struct rmap_iterator iter;
         int young = 0;
 
         /*
@@ -1197,25 +1241,24 @@ static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
         if (!shadow_accessed_mask)
                 return kvm_unmap_rmapp(kvm, rmapp, data);
 
-        spte = rmap_next(rmapp, NULL);
-        while (spte) {
-                int _young;
-                u64 _spte = *spte;
-                BUG_ON(!(_spte & PT_PRESENT_MASK));
-                _young = _spte & PT_ACCESSED_MASK;
-                if (_young) {
+        for (sptep = rmap_get_first(*rmapp, &iter); sptep;
+             sptep = rmap_get_next(&iter)) {
+                BUG_ON(!(*sptep & PT_PRESENT_MASK));
+
+                if (*sptep & PT_ACCESSED_MASK) {
                         young = 1;
-                        clear_bit(PT_ACCESSED_SHIFT, (unsigned long *)spte);
+                        clear_bit(PT_ACCESSED_SHIFT, (unsigned long *)sptep);
                 }
-                spte = rmap_next(rmapp, spte);
         }
+
         return young;
 }
 
 static int kvm_test_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
                               unsigned long data)
 {
-        u64 *spte;
+        u64 *sptep;
+        struct rmap_iterator iter;
         int young = 0;
 
         /*
@@ -1226,16 +1269,14 @@ static int kvm_test_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
         if (!shadow_accessed_mask)
                 goto out;
 
-        spte = rmap_next(rmapp, NULL);
-        while (spte) {
-                u64 _spte = *spte;
-                BUG_ON(!(_spte & PT_PRESENT_MASK));
-                young = _spte & PT_ACCESSED_MASK;
-                if (young) {
+        for (sptep = rmap_get_first(*rmapp, &iter); sptep;
+             sptep = rmap_get_next(&iter)) {
+                BUG_ON(!(*sptep & PT_PRESENT_MASK));
+
+                if (*sptep & PT_ACCESSED_MASK) {
                         young = 1;
                         break;
                 }
-                spte = rmap_next(rmapp, spte);
         }
 out:
         return young;
@@ -1887,10 +1928,11 @@ static void kvm_mmu_put_page(struct kvm_mmu_page *sp, u64 *parent_pte)
 
 static void kvm_mmu_unlink_parents(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
-        u64 *parent_pte;
+        u64 *sptep;
+        struct rmap_iterator iter;
 
-        while ((parent_pte = pte_list_next(&sp->parent_ptes, NULL)))
-                drop_parent_pte(sp, parent_pte);
+        while ((sptep = rmap_get_first(sp->parent_ptes, &iter)))
+                drop_parent_pte(sp, sptep);
 }
 
 static int mmu_zap_unsync_children(struct kvm *kvm,
diff --git a/arch/x86/kvm/mmu_audit.c b/arch/x86/kvm/mmu_audit.c
index 715da5a19a5b..7d7d0b9e23eb 100644
--- a/arch/x86/kvm/mmu_audit.c
+++ b/arch/x86/kvm/mmu_audit.c
@@ -192,7 +192,8 @@ static void audit_write_protection(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
         struct kvm_memory_slot *slot;
         unsigned long *rmapp;
-        u64 *spte;
+        u64 *sptep;
+        struct rmap_iterator iter;
 
         if (sp->role.direct || sp->unsync || sp->role.invalid)
                 return;
@@ -200,13 +201,12 @@ static void audit_write_protection(struct kvm *kvm, struct kvm_mmu_page *sp)
         slot = gfn_to_memslot(kvm, sp->gfn);
         rmapp = &slot->rmap[sp->gfn - slot->base_gfn];
 
-        spte = rmap_next(rmapp, NULL);
-        while (spte) {
-                if (is_writable_pte(*spte))
+        for (sptep = rmap_get_first(*rmapp, &iter); sptep;
+             sptep = rmap_get_next(&iter)) {
+                if (is_writable_pte(*sptep))
                         audit_printk(kvm, "shadow page has writable "
                                      "mappings: gfn %llx role %x\n",
                                      sp->gfn, sp->role.word);
-                spte = rmap_next(rmapp, spte);
         }
 }