Xen devel - Jun 2011 - problem in accessing dom0 page from domU PVHVM guest

Hi,

I could successfully access domU page from dom0 but when i tried accessing
dom0 page from domU the grant table operations are successful but when i
tried to print the contents of the mapped page, it crashes with page fault
message "BUG: unable to handle kernel paging request at
ffffc90000590000".
Please find below the relevant domU module source and the crash dump.

Can anyone please help me in fixing this?

Thanks in Advance.

Regards,
VSR.

======= domU code =======        struct vm_struct *v_start;

        printk("\nxen: domU: init_module with gref = %d", gref);
<<-- gref
(grant reference is passed as a module parameter)

        // The following function reserves a range of kernel address space
and
        // allocates pagetables to map that range. No actual mappings are
created.
        v_start = *alloc_vm_area*(PAGE_SIZE);
        if (v_start == 0) {
                free_vm_area(v_start);
                printk("\nxen: dom0: could not allocate page");
                return -EFAULT;
        }

     * gnttab_set_map_op*(&ops, (unsigned long)v_start->addr, *
GNTMAP_host_map*, gref, 0); /* flags, ref, domID */

      if (*HYPERVISOR_grant_table_op*(GNTTABOP_map_grant_ref, &ops, 1)) {
            printk("\nxen: domU: HYPERVISOR map grant ref failed");
            return -EFAULT;
      }

      if (ops.status) {
            printk("\nxen: domU: HYPERVISOR map grant ref failed status
%d",
                        ops.status);
      }
      printk("\nxen: domU: shared_page = %p, handle = %x, status =
%x",
                  v_start->addr, ops.handle, ops.status);

      printk("\nContents of the mapped page ");
      for(i=0;i<=10;i++)
      {
            printk("%c", ((char*)(v_start->addr))[i]);  
<<<--- *crashes
here*
      }
=========  crash message (on DomU)
========root@PVHVM-domU:~/test_programs/page_share_interdomain# dmesg -c

[16259.922417] xen: domU: init_module with gref = 9   <<<<<---
grant ref
passes as module param
[16259.928763] xen: domU: shared_page = ffffc90000590000, handle = 3, status
= 0  <<<<<--- the status code shows SUCCESS
[16259.928871] Contents of the mapped page                
<<<<<<<<<<---
trying to print the bytes in page; crash !!!!
[16259.929126] BUG: unable to handle kernel paging request at
ffffc90000590000
[16259.929130] IP: [<ffffffffa010b185>] init_module+0x155/0x17c
[domu_share2]
[16259.929144] PGD 3fd25067 PUD 3fd26067 PMD 3e667067 PTE 0
[16259.929178] Oops: 0000 [#3] SMP
==========================

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