[PATCH 4 of 4] xen/pagetables: Document pt_base inconsistency when running in COMPAT mode

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

# HG changeset patch
# User Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
# Date 1345579709 14400
# Node ID b992f8e613a3401b9ddd140ce436c840d412beb7
# Parent  74bedb086c5b72447262e087c0218b89f8bc9140
xen/pagetables: Document pt_base inconsistency when running in COMPAT mode.

c/s 13257 added the COMPAT mode wherein a 64-bit hypervisor can
run with a 32-bit initial domain. One of the things it added was
that the pt_base is offset by two pages. This inconsistency
is only present in the COMPAT mode so lets document it.

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

diff -r 74bedb086c5b -r b992f8e613a3 xen/include/public/xen.h
--- a/xen/include/public/xen.h	Tue Aug 21 16:08:29 2012 -0400
+++ b/xen/include/public/xen.h	Tue Aug 21 16:08:29 2012 -0400
@@ -663,7 +663,7 @@ typedef struct shared_info shared_info_t
  *      c. list of allocated page frames [mfn_list, nr_pages]
  *         (unless relocated due to XEN_ELFNOTE_INIT_P2M)
  *      d. start_info_t structure        [register ESI (x86)]
- *      e. bootstrap page tables         [pt_base, CR3 (x86)]
+ *      e. bootstrap page tables         [pt_base, CR3 (x86)] *1
  *      f. bootstrap stack               [register ESP (x86)]
  *  4. Bootstrap elements are packed together, but each is 4kB-aligned.
  *  5. The initial ram disk may be omitted.
@@ -678,6 +678,9 @@ typedef struct shared_info shared_info_t
  *
  *  NOTE: The initial virtual region (3a -> 3f) are all mapped by the initial
  *  pagetables [pt_base, CR3 (x86)].
+ *
+ *  *1: When booting under a 64-bit hypervisor with a 32-bit initial domain
+ *  it is offset by two pages (pt_base += PAGE_SIZE*2).
  */
 
 #define MAX_GUEST_CMDLINE 1024