KVM: Optimize gfn lookup in kvm_zap_gfn_range()

Introduce a memslots gfn upper bound operation and use it to optimize
kvm_zap_gfn_range().
This way this handler can do a quick lookup for intersecting gfns and won't
have to do a linear scan of the whole memslot set.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <ef242146a87a335ee93b441dcf01665cb847c902.1638817641.git.maciej.szmigiero@oracle.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index c614309..b83ae48 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5728,19 +5728,22 @@ static bool __kvm_zap_rmaps(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
 {
        const struct kvm_memory_slot *memslot;
        struct kvm_memslots *slots;
+       struct kvm_memslot_iter iter;
        bool flush = false;
        gfn_t start, end;
-       int i, bkt;
+       int i;
 
        if (!kvm_memslots_have_rmaps(kvm))
                return flush;
 
        for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
                slots = __kvm_memslots(kvm, i);
-               kvm_for_each_memslot(memslot, bkt, slots) {
+
+               kvm_for_each_memslot_in_gfn_range(&iter, slots, gfn_start, gfn_end) {
+                       memslot = iter.slot;
                        start = max(gfn_start, memslot->base_gfn);
                        end = min(gfn_end, memslot->base_gfn + memslot->npages);
-                       if (start >= end)
+                       if (WARN_ON_ONCE(start >= end))
                                continue;
 
                        flush = slot_handle_level_range(kvm, memslot, kvm_zap_rmapp,
@@ -5761,6 +5764,9 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
        bool flush;
        int i;
 
+       if (WARN_ON_ONCE(gfn_end <= gfn_start))
+               return;
+
        write_lock(&kvm->mmu_lock);
 
        kvm_inc_notifier_count(kvm, gfn_start, gfn_end);

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9eda8a6..3bc9849 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -846,6 +846,100 @@ struct kvm_memory_slot *id_to_memslot(struct kvm_memslots *slots, int id)
        return NULL;
 }
 
+/* Iterator used for walking memslots that overlap a gfn range. */
+struct kvm_memslot_iter {
+       struct kvm_memslots *slots;
+       struct rb_node *node;
+       struct kvm_memory_slot *slot;
+};
+
+static inline void kvm_memslot_iter_next(struct kvm_memslot_iter *iter)
+{
+       iter->node = rb_next(iter->node);
+       if (!iter->node)
+               return;
+
+       iter->slot = container_of(iter->node, struct kvm_memory_slot, gfn_node[iter->slots->node_idx]);
+}
+
+static inline void kvm_memslot_iter_start(struct kvm_memslot_iter *iter,
+                                         struct kvm_memslots *slots,
+                                         gfn_t start)
+{
+       int idx = slots->node_idx;
+       struct rb_node *tmp;
+       struct kvm_memory_slot *slot;
+
+       iter->slots = slots;
+
+       /*
+        * Find the so called "upper bound" of a key - the first node that has
+        * its key strictly greater than the searched one (the start gfn in our case).
+        */
+       iter->node = NULL;
+       for (tmp = slots->gfn_tree.rb_node; tmp; ) {
+               slot = container_of(tmp, struct kvm_memory_slot, gfn_node[idx]);
+               if (start < slot->base_gfn) {
+                       iter->node = tmp;
+                       tmp = tmp->rb_left;
+               } else {
+                       tmp = tmp->rb_right;
+               }
+       }
+
+       /*
+        * Find the slot with the lowest gfn that can possibly intersect with
+        * the range, so we'll ideally have slot start <= range start
+        */
+       if (iter->node) {
+               /*
+                * A NULL previous node means that the very first slot
+                * already has a higher start gfn.
+                * In this case slot start > range start.
+                */
+               tmp = rb_prev(iter->node);
+               if (tmp)
+                       iter->node = tmp;
+       } else {
+               /* a NULL node below means no slots */
+               iter->node = rb_last(&slots->gfn_tree);
+       }
+
+       if (iter->node) {
+               iter->slot = container_of(iter->node, struct kvm_memory_slot, gfn_node[idx]);
+
+               /*
+                * It is possible in the slot start < range start case that the
+                * found slot ends before or at range start (slot end <= range start)
+                * and so it does not overlap the requested range.
+                *
+                * In such non-overlapping case the next slot (if it exists) will
+                * already have slot start > range start, otherwise the logic above
+                * would have found it instead of the current slot.
+                */
+               if (iter->slot->base_gfn + iter->slot->npages <= start)
+                       kvm_memslot_iter_next(iter);
+       }
+}
+
+static inline bool kvm_memslot_iter_is_valid(struct kvm_memslot_iter *iter, gfn_t end)
+{
+       if (!iter->node)
+               return false;
+
+       /*
+        * If this slot starts beyond or at the end of the range so does
+        * every next one
+        */
+       return iter->slot->base_gfn < end;
+}
+
+/* Iterate over each memslot at least partially intersecting [start, end) range */
+#define kvm_for_each_memslot_in_gfn_range(iter, slots, start, end)     \
+       for (kvm_memslot_iter_start(iter, slots, start);                \
+            kvm_memslot_iter_is_valid(iter, end);                      \
+            kvm_memslot_iter_next(iter))
+
 /*
  * KVM_SET_USER_MEMORY_REGION ioctl allows the following operations:
  * - create a new memory slot