KVM: Minor memory slot optimization

If a slot is removed or moved in the guest physical address space, we
first allocate and install a new slot array with the invalidated
entry.  The old array is then freed.  We then proceed to allocate yet
another slot array to install the permanent replacement.  Re-use the
original array when this occurs and avoid the extra kfree/kmalloc.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 14 insertions, 7 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9a56ca2fa257..4c8c0657e370 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -716,7 +716,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
         unsigned long npages;
         struct kvm_memory_slot *memslot, *slot;
         struct kvm_memory_slot old, new;
-        struct kvm_memslots *slots, *old_memslots;
+        struct kvm_memslots *slots = NULL, *old_memslots;
 
         r = check_memory_region_flags(mem);
         if (r)
@@ -832,18 +832,25 @@ int __kvm_set_memory_region(struct kvm *kvm,
                  *      - kvm_is_visible_gfn (mmu_check_roots)
                  */
                 kvm_arch_flush_shadow_memslot(kvm, slot);
-                kfree(old_memslots);
+                slots = old_memslots;
         }
 
         r = kvm_arch_prepare_memory_region(kvm, &new, old, mem, user_alloc);
         if (r)
-                goto out_free;
+                goto out_slots;
 
         r = -ENOMEM;
-        slots = kmemdup(kvm->memslots, sizeof(struct kvm_memslots),
-                        GFP_KERNEL);
-        if (!slots)
-                goto out_free;
+        /*
+         * We can re-use the old_memslots from above, the only difference
+         * from the currently installed memslots is the invalid flag.  This
+         * will get overwritten by update_memslots anyway.
+         */
+        if (!slots) {
+                slots = kmemdup(kvm->memslots, sizeof(struct kvm_memslots),
+                                GFP_KERNEL);
+                if (!slots)
+                        goto out_free;
+        }
 
         /* map new memory slot into the iommu */
         if (npages) {