KVM: Clean up local variable usage in __kvm_set_memory_region()

Clean up __kvm_set_memory_region() to achieve several goals:

  - Remove local variables that serve no real purpose
  - Improve the readability of the code
  - Better show the relationship between the 'old' and 'new' memslot
  - Prepare for dynamically sizing memslots
  - Document subtle gotchas (via comments)

Note, using 'tmp' to hold the initial memslot is not strictly necessary
at this juncture, e.g. 'old' could be directly copied from
id_to_memslot(), but keep the pointer usage as id_to_memslot() will be
able to return a NULL pointer once memslots are dynamically sized.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 642215e..1632e46 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1071,13 +1071,11 @@ static int kvm_delete_memslot(struct kvm *kvm,
 int __kvm_set_memory_region(struct kvm *kvm,
                            const struct kvm_userspace_memory_region *mem)
 {
-       int r;
-       gfn_t base_gfn;
-       unsigned long npages;
-       struct kvm_memory_slot *slot;
        struct kvm_memory_slot old, new;
-       int as_id, id;
+       struct kvm_memory_slot *tmp;
        enum kvm_mr_change change;
+       int as_id, id;
+       int r;
 
        r = check_memory_region_flags(mem);
        if (r)
@@ -1102,54 +1100,58 @@ int __kvm_set_memory_region(struct kvm *kvm,
        if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
                return -EINVAL;
 
-       slot = id_to_memslot(__kvm_memslots(kvm, as_id), id);
-       base_gfn = mem->guest_phys_addr >> PAGE_SHIFT;
-       npages = mem->memory_size >> PAGE_SHIFT;
-
-       if (npages > KVM_MEM_MAX_NR_PAGES)
-               return -EINVAL;
-
        /*
         * Make a full copy of the old memslot, the pointer will become stale
         * when the memslots are re-sorted by update_memslots(), and the old
         * memslot needs to be referenced after calling update_memslots(), e.g.
-        * to free its resources and for arch specific behavior.
+        * to free its resources and for arch specific behavior.  Kill @tmp
+        * after making a copy to deter potentially dangerous usage.
         */
-       old = *slot;
+       tmp = id_to_memslot(__kvm_memslots(kvm, as_id), id);
+       old = *tmp;
+       tmp = NULL;
+
        if (!mem->memory_size)
                return kvm_delete_memslot(kvm, mem, &old, as_id);
 
-       new = old;
-
        new.id = id;
-       new.base_gfn = base_gfn;
-       new.npages = npages;
+       new.base_gfn = mem->guest_phys_addr >> PAGE_SHIFT;
+       new.npages = mem->memory_size >> PAGE_SHIFT;
        new.flags = mem->flags;
        new.userspace_addr = mem->userspace_addr;
 
+       if (new.npages > KVM_MEM_MAX_NR_PAGES)
+               return -EINVAL;
+
        if (!old.npages) {
                change = KVM_MR_CREATE;
+               new.dirty_bitmap = NULL;
+               memset(&new.arch, 0, sizeof(new.arch));
        } else { /* Modify an existing slot. */
                if ((new.userspace_addr != old.userspace_addr) ||
-                   (npages != old.npages) ||
+                   (new.npages != old.npages) ||
                    ((new.flags ^ old.flags) & KVM_MEM_READONLY))
                        return -EINVAL;
 
-               if (base_gfn != old.base_gfn)
+               if (new.base_gfn != old.base_gfn)
                        change = KVM_MR_MOVE;
                else if (new.flags != old.flags)
                        change = KVM_MR_FLAGS_ONLY;
                else /* Nothing to change. */
                        return 0;
+
+               /* Copy dirty_bitmap and arch from the current memslot. */
+               new.dirty_bitmap = old.dirty_bitmap;
+               memcpy(&new.arch, &old.arch, sizeof(new.arch));
        }
 
        if ((change == KVM_MR_CREATE) || (change == KVM_MR_MOVE)) {
                /* Check for overlaps */
-               kvm_for_each_memslot(slot, __kvm_memslots(kvm, as_id)) {
-                       if (slot->id == id)
+               kvm_for_each_memslot(tmp, __kvm_memslots(kvm, as_id)) {
+                       if (tmp->id == id)
                                continue;
-                       if (!((base_gfn + npages <= slot->base_gfn) ||
-                             (base_gfn >= slot->base_gfn + slot->npages)))
+                       if (!((new.base_gfn + new.npages <= tmp->base_gfn) ||
+                             (new.base_gfn >= tmp->base_gfn + tmp->npages)))
                                return -EEXIST;
                }
        }