KVM: Resync only arch fields when slots_arch_lock gets reacquired

There is no need to copy the whole memslot data after releasing
slots_arch_lock for a moment to install temporary memslots copy in
kvm_set_memslot() since this lock only protects the arch field of each
memslot.

Just resync this particular field after reacquiring slots_arch_lock.

Note, this also eliminates the need to manually clear the INVALID flag
when restoring memslots; the "setting" of the INVALID flag was an
unwanted side effect of copying the entire memslots.

Since kvm_copy_memslots() has just one caller remaining now
open-code it instead.
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
[sean: tweak shortlog, note INVALID flag in changelog, revert comment]
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <b63035d114707792e9042f074478337f770dff6a.1638817638.git.maciej.szmigiero@oracle.com>

virt/kvm/kvm_main.c

@@ -1515,12 +1515,6 @@ static size_t kvm_memslots_size(int slots)
 	       (sizeof(struct kvm_memory_slot) * slots);
 }
 
-static void kvm_copy_memslots(struct kvm_memslots *to,
-			      struct kvm_memslots *from)
-{
-	memcpy(to, from, kvm_memslots_size(from->used_slots));
-}
-
 /*
  * 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
@@ -1539,11 +1533,22 @@ static struct kvm_memslots *kvm_dup_memslots(struct kvm_memslots *old,
 
 	slots = kvzalloc(new_size, GFP_KERNEL_ACCOUNT);
 	if (likely(slots))
-		kvm_copy_memslots(slots, old);
+		memcpy(slots, old, kvm_memslots_size(old->used_slots));
 
 	return slots;
 }
 
+static void kvm_copy_memslots_arch(struct kvm_memslots *to,
+				   struct kvm_memslots *from)
+{
+	int i;
+
+	WARN_ON_ONCE(to->used_slots != from->used_slots);
+
+	for (i = 0; i < from->used_slots; i++)
+		to->memslots[i].arch = from->memslots[i].arch;
+}
+
 static int kvm_set_memslot(struct kvm *kvm,
 			   const struct kvm_userspace_memory_region *mem,
 			   struct kvm_memory_slot *new, int as_id,
@@ -1584,9 +1589,10 @@ static int kvm_set_memslot(struct kvm *kvm,
 		slot->flags |= KVM_MEMSLOT_INVALID;
 
 		/*
-		 * We can re-use the memory from the old memslots.
-		 * It will be overwritten with a copy of the new memslots
-		 * after reacquiring the slots_arch_lock below.
+		 * We can re-use the old memslots, the only difference from the
+		 * newly installed memslots is the invalid flag, which will get
+		 * dropped by update_memslots anyway.  We'll also revert to the
+		 * old memslots if preparing the new memory region fails.
 		 */
 		slots = install_new_memslots(kvm, as_id, slots);
 
@@ -1603,12 +1609,14 @@ static int kvm_set_memslot(struct kvm *kvm,
 		mutex_lock(&kvm->slots_arch_lock);
 
 		/*
-		 * The arch-specific fields of the memslots could have changed
-		 * between releasing the slots_arch_lock in
-		 * install_new_memslots and here, so get a fresh copy of the
-		 * slots.
+		 * The arch-specific fields of the now-active memslots could
+		 * have been modified between releasing slots_arch_lock in
+		 * install_new_memslots and re-acquiring slots_arch_lock above.
+		 * Copy them to the inactive memslots.  Arch code is required
+		 * to retrieve memslots *after* acquiring slots_arch_lock, thus
+		 * the active memslots are guaranteed to be fresh.
 		 */
-		kvm_copy_memslots(slots, __kvm_memslots(kvm, as_id));
+		kvm_copy_memslots_arch(slots, __kvm_memslots(kvm, as_id));
 	}
 
 	/*
@@ -1657,13 +1665,10 @@ static int kvm_set_memslot(struct kvm *kvm,
 	return 0;
 
 out_slots:
-	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
-		slot = id_to_memslot(slots, new->id);
-		slot->flags &= ~KVM_MEMSLOT_INVALID;
+	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE)
 		slots = install_new_memslots(kvm, as_id, slots);
-	} else {
+	else
 		mutex_unlock(&kvm->slots_arch_lock);
-	}
 	kvfree(slots);
 	return r;
 }