drivers: xen: gnttab: process gnttabs in reverse order

The Xen extends domain grant tables every time domain requests gnttab
basing on gnttab idx. If idx > xen_current_max_gnttab_idx the Xen extends
grant table so that idx <= xen_current_max_gnttab_idx. The growing grant
tables on every hypercall is a bit costly operation and it also results in
the bunch of log messages:

(XEN) xen-source/xen/common/grant_table.c:1909:d0v0 Expanding d0 \
  grant table from 1 to 2 frames

This patch changes gnttab processing from gnttab max_idx to low_idx, so the
first hypercall has the largest index, ensuring that the grant table will
grow only once. It also reduces number of log messages.

Signed-off-by: Grygorii Strashko <[email protected]>
Acked-by: Volodymyr Babchuk <[email protected]>
Reviewed-by: Dmytro Firsov <[email protected]>

drivers/xen/gnttab.c

@@ -372,12 +372,12 @@ static int gnttab_init(void)
 		gnttab.gref_list[0] = gref;
 	}
 
-	for (i = 0; i < CONFIG_NR_GRANT_FRAMES; i++) {
+	for (i = CONFIG_NR_GRANT_FRAMES; i; i--) {
 		xatp.domid = DOMID_SELF;
 		xatp.size = 0;
 		xatp.space = XENMAPSPACE_grant_table;
-		xatp.idx = i;
-		xatp.gpfn = xen_virt_to_gfn(gnttab_base) + i;
+		xatp.idx = i - 1;
+		xatp.gpfn = xen_virt_to_gfn(gnttab_base) + (i - 1);
 		rc = HYPERVISOR_memory_op(XENMEM_add_to_physmap, &xatp);
 		__ASSERT(!rc, "add_to_physmap failed; status = %d\n", rc);
 	}