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>

drivers/xen/grant-table.c

@@ -120,6 +120,17 @@ struct gnttab_ops {
 	 * by bit operations.
 	 */
 	int (*query_foreign_access)(grant_ref_t ref);
+	/*
+	 * Grant a domain to access a range of bytes within the page referred by
+	 * an available grant entry. Ref parameter is reference of a grant entry
+	 * which will be sub-page accessed, domid is id of grantee domain, frame
+	 * is frame address of subpage grant, flags is grant type and flag
+	 * information, page_off is offset of the range of bytes, and length is
+	 * length of bytes to be accessed.
+	 */
+	void (*update_subpage_entry)(grant_ref_t ref, domid_t domid,
+				     unsigned long frame, int flags,
+				     unsigned page_off, unsigned length);
 };
 
 static struct gnttab_ops *gnttab_interface;
@@ -261,6 +272,66 @@ int gnttab_grant_foreign_access(domid_t domid, unsigned long frame,
 }
 EXPORT_SYMBOL_GPL(gnttab_grant_foreign_access);
 
+void gnttab_update_subpage_entry_v2(grant_ref_t ref, domid_t domid,
+				    unsigned long frame, int flags,
+				    unsigned page_off,
+				    unsigned length)
+{
+	gnttab_shared.v2[ref].sub_page.frame = frame;
+	gnttab_shared.v2[ref].sub_page.page_off = page_off;
+	gnttab_shared.v2[ref].sub_page.length = length;
+	gnttab_shared.v2[ref].hdr.domid = domid;
+	wmb();
+	gnttab_shared.v2[ref].hdr.flags =
+				GTF_permit_access | GTF_sub_page | flags;
+}
+
+int gnttab_grant_foreign_access_subpage_ref(grant_ref_t ref, domid_t domid,
+					    unsigned long frame, int flags,
+					    unsigned page_off,
+					    unsigned length)
+{
+	if (flags & (GTF_accept_transfer | GTF_reading |
+		     GTF_writing | GTF_transitive))
+		return -EPERM;
+
+	if (gnttab_interface->update_subpage_entry == NULL)
+		return -ENOSYS;
+
+	gnttab_interface->update_subpage_entry(ref, domid, frame, flags,
+					       page_off, length);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(gnttab_grant_foreign_access_subpage_ref);
+
+int gnttab_grant_foreign_access_subpage(domid_t domid, unsigned long frame,
+					int flags, unsigned page_off,
+					unsigned length)
+{
+	int ref, rc;
+
+	ref = get_free_entries(1);
+	if (unlikely(ref < 0))
+		return -ENOSPC;
+
+	rc = gnttab_grant_foreign_access_subpage_ref(ref, domid, frame, flags,
+						     page_off, length);
+	if (rc < 0) {
+		put_free_entry(ref);
+		return rc;
+	}
+
+	return ref;
+}
+EXPORT_SYMBOL_GPL(gnttab_grant_foreign_access_subpage);
+
+bool gnttab_subpage_grants_available(void)
+{
+	return gnttab_interface->update_subpage_entry != NULL;
+}
+EXPORT_SYMBOL_GPL(gnttab_subpage_grants_available);
+
 static int gnttab_query_foreign_access_v1(grant_ref_t ref)
 {
 	return gnttab_shared.v1[ref].flags & (GTF_reading|GTF_writing);
@@ -813,6 +884,7 @@ static struct gnttab_ops gnttab_v2_ops = {
 	.end_foreign_access_ref		= gnttab_end_foreign_access_ref_v2,
 	.end_foreign_transfer_ref	= gnttab_end_foreign_transfer_ref_v2,
 	.query_foreign_access		= gnttab_query_foreign_access_v2,
+	.update_subpage_entry		= gnttab_update_subpage_entry_v2,
 };
 
 static void gnttab_request_version(void)

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);