domctl.h: document non-standard error codes for enabling paging/access

The domctl to enable paging and access returns some non-standard error
codes after failure. This can be used in the tools to print specific
error messages. xenpaging recognizes these errno values and shows them
if the init function fails.

Document the return codes in the public header file.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>

xen/include/public/domctl.h

@@ -716,6 +716,13 @@ struct xen_domctl_gdbsx_domstatus {
  * Domctl interface to set up and tear down the 
  * pager<->hypervisor interface. Use XENMEM_paging_op*
  * to perform per-page operations.
+ *
+ * The XEN_DOMCTL_MEM_EVENT_OP_PAGING_ENABLE domctl returns several
+ * non-standard error codes to indicate why paging could not be enabled:
+ * ENODEV - host lacks HAP support (EPT/NPT) or HAP is disabled in guest
+ * EMLINK - guest has iommu passthrough enabled
+ * EXDEV  - guest has PoD enabled
+ * EBUSY  - guest has or had paging enabled, ring buffer still active
  */
 #define XEN_DOMCTL_MEM_EVENT_OP_PAGING            1
 
@@ -735,6 +742,11 @@ struct xen_domctl_gdbsx_domstatus {
  *
  * The memory event handler can then resume the VCPU and redo the access 
  * with a XENMEM_access_op_resume hypercall.
+ *
+ * The XEN_DOMCTL_MEM_EVENT_OP_ACCESS_ENABLE domctl returns several
+ * non-standard error codes to indicate why access could not be enabled:
+ * ENODEV - host lacks HAP support (EPT/NPT) or HAP is disabled in guest
+ * EBUSY  - guest has or had access enabled, ring buffer still active
  */
 #define XEN_DOMCTL_MEM_EVENT_OP_ACCESS            2