Mini-OS: new_pt_frame fails

[Julian Stecklina <der_julian@web.de>]

Hello,

as some might have read in my previous postings, I have a Mini-OS dom0
that constructs a Mini-OS domU on x86 xen-3.0.4-testing. As the memory
layout my Dom0 builds does not guarantee, that pages the DomU Mini-OS 
tries to
use as page tables are mapped to existing memory, I changed new_pt_frame
in arch/x86/mm.c in the following way:

The pt_pfn is mapped R/W, then it is zeroed by a memset(). After that it
is mapped with the correct protection flags and pinned. The problem is
that the first mapping fails:

Mapping memory range 0xc0400000 - 0xc1000000
MINI_OS(file=mm.c, line=68) Allocating new L1 pt frame for pt_pfn=36,
mfn=ca8, prev_l_mfn=cad, offset=301
(XEN) .../xen-3.0.4-testing.hg/xen/include/asm/mm.h:184:d1 Error pfn 0:
rd=ffbe8100, od=ffbf4100, caf=80000002, taf=e8000002
(XEN) mm.c:456:d1 Could not get page ref for pfn 0
(XEN) mm.c:2225:d1 Could not get page for normal update
New PT could not be mapped R/W.

Now I am puzzled what these hypervisor errors mean, and what I could 
have done wrong? The code in question is the following:

     mmu_updates[0].ptr = ((pgentry_t)tab[l2_table_offset(pt_page)] &
PAGE_MASK) +
                          sizeof(pgentry_t) * l1_table_offset(pt_page);
     mmu_updates[0].val = ((pgentry_t)pfn_to_mfn(*pt_pfn) << PAGE_SHIFT) 
| _PAGE_RW | _PAGE_PRESENT;
     if(HYPERVISOR_mmu_update(mmu_updates, 1, NULL, DOMID_SELF) < 0)
     {
          printk("New PT could not be mapped R/W.\n");
          do_exit();
     }


Any help is appreciated.

Regards,
Julian