libxc: Replace malloc with alloca in hot path

Replace malloc with alloc in hot paths for improved performance.

Signed-off-by: Santosh Jodh <santosh.jodh@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxc/xc_linux_osdep.c

@@ -243,7 +243,7 @@ static void *linux_privcmd_map_foreign_bulk(xc_interface *xch, xc_osdep_handle h
          * IOCTL_PRIVCMD_MMAPBATCH_V2 is not supported - fall back to
          * IOCTL_PRIVCMD_MMAPBATCH.
          */
-        xen_pfn_t *pfn = malloc(num * sizeof(*pfn));
+        xen_pfn_t *pfn = alloca(num * sizeof(*pfn));
 
         if ( pfn )
         {
@@ -289,8 +289,6 @@ static void *linux_privcmd_map_foreign_bulk(xc_interface *xch, xc_osdep_handle h
                 break;
             }
 
-            free(pfn);
-
             if ( rc == -ENOENT && i == num )
                 rc = 0;
             else if ( rc )
@@ -525,7 +523,7 @@ static void *linux_gnttab_grant_map(xc_gnttab *xch, xc_osdep_handle h,
     if (flags & XC_GRANT_MAP_SINGLE_DOMAIN)
         domids_stride = 0;
 
-    map = malloc(sizeof(*map) +
+    map = alloca(sizeof(*map) +
                  (count - 1) * sizeof(struct ioctl_gntdev_map_grant_ref));
     if ( map == NULL )
         return NULL;
@@ -599,7 +597,6 @@ static void *linux_gnttab_grant_map(xc_gnttab *xch, xc_osdep_handle h,
     }
 
  out:
-    free(map);
 
     return addr;
 }