KVM: Cache as_id in kvm_memory_slot

Cache the address space ID just like the slot ID.  It will be used in
order to fill in the dirty ring entries.
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201014182700.2888246-7-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/kvm_host.h

@@ -346,6 +346,7 @@ struct kvm_memory_slot {
 	unsigned long userspace_addr;
 	u32 flags;
 	short id;
+	u16 as_id;
 };
 
 static inline unsigned long kvm_dirty_bitmap_bytes(struct kvm_memory_slot *memslot)

virt/kvm/kvm_main.c

@@ -1247,6 +1247,11 @@ static int kvm_delete_memslot(struct kvm *kvm,
 
 	memset(&new, 0, sizeof(new));
 	new.id = old->id;
+	/*
+	 * This is only for debugging purpose; it should never be referenced
+	 * for a removed memslot.
+	 */
+	new.as_id = as_id;
 
 	r = kvm_set_memslot(kvm, mem, old, &new, as_id, KVM_MR_DELETE);
 	if (r)
@@ -1313,6 +1318,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	if (!mem->memory_size)
 		return kvm_delete_memslot(kvm, mem, &old, as_id);
 
+	new.as_id = as_id;
 	new.id = id;
 	new.base_gfn = mem->guest_phys_addr >> PAGE_SHIFT;
 	new.npages = mem->memory_size >> PAGE_SHIFT;