xen/granttable: Support sub-page grants

- They can't be used to map the page (so can only be used in a GNTTABOP_copy
  hypercall).
- It's possible to grant access with a finer granularity than whole pages.
- Xen guarantees that they can be revoked quickly (a normal map grant can
  only be revoked with the cooperation of the domain which has been granted
  access).

Signed-off-by: Annie Li <annie.li@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

include/xen/grant_table.h

@@ -62,6 +62,15 @@ int gnttab_resume(void);
 
 int gnttab_grant_foreign_access(domid_t domid, unsigned long frame,
 				int readonly);
+int gnttab_grant_foreign_access_subpage(domid_t domid, unsigned long frame,
+					int flags, unsigned page_off,
+					unsigned length);
+
+/*
+ * Are sub-page grants available on this version of Xen?  Returns true if they
+ * are, and false if they're not.
+ */
+bool gnttab_subpage_grants_available(void);
 
 /*
  * End access through the given grant reference, iff the grant entry is no
@@ -108,6 +117,10 @@ void gnttab_cancel_free_callback(struct gnttab_free_callback *callback);
 
 void gnttab_grant_foreign_access_ref(grant_ref_t ref, domid_t domid,
 				     unsigned long frame, int readonly);
+int gnttab_grant_foreign_access_subpage_ref(grant_ref_t ref, domid_t domid,
+					    unsigned long frame, int flags,
+					    unsigned page_off,
+					    unsigned length);
 
 void gnttab_grant_foreign_transfer_ref(grant_ref_t, domid_t domid,
 				       unsigned long pfn);