[PATCH v3 15/15] KVM: Dynamically size memslot array based on number of used slots

[Sean Christopherson]

Now that the memslot logic doesn't assume memslots are always non-NULL,
dynamically size the array of memslots instead of unconditionally
allocating memory for the maximum number of memslots.

Note, because a to-be-deleted memslot must first be invalidated, the
array size cannot be immediately reduced when deleting a memslot.
However, consecutive deletions will realize the memory savings, i.e.
a second deletion will trim the entry.

Tested-by: Christoffer Dall <christoffer.d...@arm.com>
Signed-off-by: Sean Christopherson <sean.j.christopher...@intel.com>
---
 include/linux/kvm_host.h |  2 +-
 virt/kvm/kvm_main.c      | 31 ++++++++++++++++++++++++++++---
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index b5d4133a11e1..2d73b6e5fe6e 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -432,11 +432,11 @@ static inline int kvm_arch_vcpu_memslots_id(struct 
kvm_vcpu *vcpu)
  */
 struct kvm_memslots {
        u64 generation;
-       struct kvm_memory_slot memslots[KVM_MEM_SLOTS_NUM];
        /* The mapping table from slot id to the index in memslots[]. */
        short id_to_index[KVM_MEM_SLOTS_NUM];
        atomic_t lru_slot;
        int used_slots;
+       struct kvm_memory_slot memslots[];
 };
 
 struct kvm {
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7bc88375cf53..7dd02ee6ef95 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -535,7 +535,7 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
                return NULL;
 
        for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
-               slots->id_to_index[i] = slots->memslots[i].id = -1;
+               slots->id_to_index[i] = -1;
 
        return slots;
 }
@@ -1015,6 +1015,32 @@ static struct kvm_memslots *install_new_memslots(struct 
kvm *kvm,
        return old_memslots;
 }
 
+/*
+ * Note, at a minimum, the current number of used slots must be allocated, even
+ * when deleting a memslot, as we need a complete duplicate of the memslots for
+ * use when invalidating a memslot prior to deleting/moving the memslot.
+ */
+static struct kvm_memslots *kvm_dup_memslots(struct kvm_memslots *old,
+                                            enum kvm_mr_change change)
+{
+       struct kvm_memslots *slots;
+       size_t old_size, new_size;
+
+       old_size = sizeof(struct kvm_memslots) +
+                  (sizeof(struct kvm_memory_slot) * old->used_slots);
+
+       if (change == KVM_MR_CREATE)
+               new_size = old_size + sizeof(struct kvm_memory_slot);
+       else
+               new_size = old_size;
+
+       slots = kvzalloc(new_size, GFP_KERNEL_ACCOUNT);
+       if (likely(slots))
+               memcpy(slots, old, old_size);
+
+       return slots;
+}
+
 static int kvm_set_memslot(struct kvm *kvm,
                           const struct kvm_userspace_memory_region *mem,
                           const struct kvm_memory_slot *old,
@@ -1025,10 +1051,9 @@ static int kvm_set_memslot(struct kvm *kvm,
        struct kvm_memslots *slots;
        int r;
 
-       slots = kvzalloc(sizeof(struct kvm_memslots), GFP_KERNEL_ACCOUNT);
+       slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change);
        if (!slots)
                return -ENOMEM;
-       memcpy(slots, __kvm_memslots(kvm, as_id), sizeof(struct kvm_memslots));
 
        if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
                /*
-- 
2.22.0

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