xen/arm: introduce XENFEAT_grant_map_identity

The flag tells us that the hypervisor maps a grant page to guest
physical address == machine address of the page in addition to the
normal grant mapping address. It is needed to properly issue cache
maintenance operation at the completion of a DMA operation involving a
foreign grant.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Denis Schneider <v1ne2go@gmail.com>

arch/arm/xen/enlighten.c

@@ -260,6 +260,12 @@ static int __init xen_guest_init(void)
 	xen_domain_type = XEN_HVM_DOMAIN;
 
 	xen_setup_features();
+
+	if (!xen_feature(XENFEAT_grant_map_identity)) {
+		pr_warn("Please upgrade your Xen.\n"
+				"If your platform has any non-coherent DMA devices, they won't work properly.\n");
+	}
+
 	if (xen_feature(XENFEAT_dom0))
 		xen_start_info->flags |= SIF_INITDOMAIN|SIF_PRIVILEGED;
 	else

include/xen/interface/features.h

@@ -53,6 +53,9 @@
 /* operation as Dom0 is supported */
 #define XENFEAT_dom0                      11
 
+/* Xen also maps grant references at pfn = mfn */
+#define XENFEAT_grant_map_identity        12
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */