Can't map the page referenced by HVM-DomU CR3 in Dom0

[david <david_n@gmx.at>]

Hi,

I'm trying to access the page containing the paging information for a 
DomU from Dom0.

I'm doing that by translating the address contained in the DomU CR3 
register with xc_translate_foreign_address (libxc) and try to map the
returned frame number with xc_map_foreign_range.

The problem is, that the return value from xc_translate_foreign_address 
is 0 (guest cr3 is 0x002f3000 in my case), which indicates an error 
(corresponding to the code comments). After some debugging I have 
discovered, that pte becomes 0 when level=2 and therefore the function 
returns 0 on line 79:

tools/libxc/xc_pagetab.c
69    /* Walk the pagetables */
70    for (level = pt_levels; level > 0; level--) {
71        paddr += ((virt & mask) >> (xc_ffs64(mask) - 1)) * size;
72        map = xc_map_foreign_range(xc_handle, dom, PAGE_SIZE, PROT_READ,
73                                   paddr >>PAGE_SHIFT);
74        if (!map)
75            return 0;
76        memcpy(&pte, map + (paddr & (PAGE_SIZE - 1)), size);
77        munmap(map, PAGE_SIZE);
78        if (!(pte & 1))
79            return 0;
80        paddr = pte & 0x000ffffffffff000ull;
...
...

I'm currently trying to examine why pte becomes 0. Is anyone familiar 
with this part of the code and can explain why it is not possible to map 
the page?

My setup:
Dom0: debian 2.6.32.26 x86_64
DomU: HVM Win XP SP2 32bit
xen-4.0.1


thanks for any hints,


david