KVM: support any-length wildcard ioeventfd

It is sometimes benefitial to ignore IO size, and only match on address.
In hindsight this would have been a better default than matching length
when KVM_IOEVENTFD_FLAG_DATAMATCH is not set, In particular, this kind
of access can be optimized on VMX: there no need to do page lookups.
This can currently be done with many ioeventfds but in a suboptimal way.

However we can't change kernel/userspace ABI without risk of breaking
some applications.
Use len = 0 to mean "ignore length for matching" in a more optimal way.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

virt/kvm/eventfd.c

@@ -600,7 +600,15 @@ ioeventfd_in_range(struct _ioeventfd *p, gpa_t addr, int len, const void *val)
 {
 	u64 _val;
 
-	if (!(addr == p->addr && len == p->length))
+	if (addr != p->addr)
+		/* address must be precise for a hit */
+		return false;
+
+	if (!p->length)
+		/* length = 0 means only look at the address, so always a hit */
+		return true;
+
+	if (len != p->length)
 		/* address-range must be precise for a hit */
 		return false;
 
@@ -671,9 +679,11 @@ ioeventfd_check_collision(struct kvm *kvm, struct _ioeventfd *p)
 
 	list_for_each_entry(_p, &kvm->ioeventfds, list)
 		if (_p->bus_idx == p->bus_idx &&
-		    _p->addr == p->addr && _p->length == p->length &&
-		    (_p->wildcard || p->wildcard ||
-		     _p->datamatch == p->datamatch))
+		    _p->addr == p->addr &&
+		    (!_p->length || !p->length ||
+		     (_p->length == p->length &&
+		      (_p->wildcard || p->wildcard ||
+		       _p->datamatch == p->datamatch))))
 			return true;
 
 	return false;
@@ -697,8 +707,9 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 	int                       ret;
 
 	bus_idx = ioeventfd_bus_from_flags(args->flags);
-	/* must be natural-word sized */
+	/* must be natural-word sized, or 0 to ignore length */
 	switch (args->len) {
+	case 0:
 	case 1:
 	case 2:
 	case 4:
@@ -716,6 +727,12 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 	if (args->flags & ~KVM_IOEVENTFD_VALID_FLAG_MASK)
 		return -EINVAL;
 
+	/* ioeventfd with no length can't be combined with DATAMATCH */
+	if (!args->len &&
+	    args->flags & (KVM_IOEVENTFD_FLAG_PIO |
+			   KVM_IOEVENTFD_FLAG_DATAMATCH))
+		return -EINVAL;
+
 	eventfd = eventfd_ctx_fdget(args->fd);
 	if (IS_ERR(eventfd))
 		return PTR_ERR(eventfd);