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Subject[PATCH v4 18/19] KVM: Dynamically size memslot array based on number of used slots
Date
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.dall@arm.com>
Tested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@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 49b6b457a157..eecfa1fe0fc7 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -433,11 +433,11 @@ 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 a1566c5cee26..57926579551b 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -569,7 +569,7 @@ 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;
}
@@ -1081,6 +1081,32 @@ 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,
struct kvm_memory_slot *old,
@@ -1091,10 +1117,9 @@ 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) {
/*
--
2.24.1
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