Xen users - Apr 2012 - Cannot map memory using grant table

Hi everyone,

I want to use grant table to transfer data by shared memory, and now want
to only share only one page. I try to learn how to use it by checking
aseemsethi''s
instruction<http://aseemsethi.wordpress.com/article/learning-grant-tables-29fizhrip655z-5/>and
gntdev.c. Although the sender seems OK, the reciever cannot map memory
by gref, and the ops.status always returns  -1, meaning undefined error.

I‘m totally confused about this. I guess it might due to memory-allocating,
but perhaps not. Could someone give some hints to me? Thanks!

Here is part of my code.

In the sender:
static int server_alloc_one_page_to_grant(unsigned long* mypage,domid_t
rdom){
        unsigned long addr=0;

        gref=-1; //gref is globle
        addr=get_zeroed_page(GFP_KERNEL);
        if(addr==0){
                printk("cannot alloc page\n");
        }

        sprintf((char*)addr,"%s\naaa\n","This is in
sender.");

        gref=gnttab_grant_foreign_access(rdom,virt_to_mfn(addr),1);
        if(gref<0){
                 free_page(addr);
        }

        printk("gref:%d\n",gref);
        return gref;
}

and it seems works well.

But in reciver:
static int client_map_page(domid_t rdom,grant_ref_t ref,struct vm_struct**
pvm){
  struct vm_struct *v_start;
  struct gnttab_map_grant_ref ops;
  struct gnttab_unmap_grant_ref unmap_ops;

  memset(&ops,0,sizeof(ops));
  memset(&ops,0,sizeof(unmap_ops));
  v_start=alloc_vm_area(PAGE_SIZE,NULL);
  if(v_start==0){
    printk("could not allocate page in client.\n");
    return -1;
  }

  /***************************things goes wrong
************************************************/
  gnttab_set_map_op(&ops,(unsigned
long)v_start->addr,GNTMAP_host_map,ref,rdom);
  if(HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref,&ops,1)){
    free_vm_area(v_start);
    printk("\nHYPERVISOR map grant ref failed\n");
    return -1;
  }
   //ops,status always equals to -1, and map failed
  if(ops.status){
    printk("xen:HYPERVISOR map grant ref failed
status=%d",ops.status);
    free_vm_area(v_start);
    return -1;
  }

  /*access memory by v_start->addr and unmap memory*/
  ...
}


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