Re: Grant tables

[Carlo Bertoldi <mcbain@tiscali.it>]

Mark Williamson wrote, on the 05/04/2008 23.14:
 > Take a look in the Xen Linux tree at the drivers/xen/blkfront/ for 
some creation of grant references.  As Samuel says, searching for grant 
table functions under drivers/xen/ is going to find you some interesting 
examples in various places.
 >
I studied the suggested code and I wrote something to test my 
understanding. It compiles and run fine, but I've got a problem: when I 
read the value of the shared structure it's not right.

This is the function, contained in a module, that creates the grant:

static void offer_page()
{
     int code;
     struct foo foo_struct;

     foo_struct.numero = 5;

     if ( ( code = gnttab_grant_foreign_access(0, 
virt_to_mfn(&foo_struct), 0)) == -1) {
         printk(KERN_INFO "grant error\n");
     }
}

In userspace I use the following code to read the granted page:

void grant(int xcg_handle, int domid)
{
     struct foo *foo_struct;
     void *buf;
     int i;

     buf = xc_gnttab_map_grant_ref(xcg_handle, domid, 851, PROT_READ); 
// yes, ugly hardcoded value

     if (buf != NULL) {
         foo_struct = (struct foo *) buf;
         printf("numero = %d\n", foo_struct->numero);
     }
}

I'd expect it to print five, but it always write zero. What am I doing 
wrong here?

 > hope that helps,
 > Cheers,
 > Mark

Thanks,
  Carlo
-- 
È molto più bello sapere qualcosa di tutto, che sapere tutto di una cosa.

Blaise Pascal