x86/gdt: Drop write-only, xalloc()'d array from set_gdt()

It is not used, and can cause a spurious failure of the set_gdt() hypercall in
low memory situations.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>
master commit: a7bd9b1661
master date: 2015-08-05 10:32:45 +0100

xen/arch/x86/mm.c

@@ -4361,20 +4361,15 @@ long set_gdt(struct vcpu *v,
     l1_pgentry_t *pl1e;
     /* NB. There are 512 8-byte entries per GDT page. */
     int i, nr_pages = (entries + 511) / 512;
-    unsigned long mfn, *pfns;
 
     if ( entries > FIRST_RESERVED_GDT_ENTRY )
         return -EINVAL;
 
-    pfns = xmalloc_array(unsigned long, nr_pages);
-    if ( !pfns )
-        return -ENOMEM;
-
     /* Check the pages in the new GDT. */
     for ( i = 0; i < nr_pages; i++ )
     {
         struct page_info *page;
-        pfns[i] = frames[i];
+
         page = get_page_from_gfn(d, frames[i], NULL, P2M_ALLOC);
         if ( !page )
             goto fail;
@@ -4383,7 +4378,7 @@ long set_gdt(struct vcpu *v,
             put_page(page);
             goto fail;
         }
-        mfn = frames[i] = page_to_mfn(page);
+        frames[i] = page_to_mfn(page);
     }
 
     /* Tear down the old GDT. */
@@ -4398,7 +4393,6 @@ long set_gdt(struct vcpu *v,
         l1e_write(&pl1e[i], l1e_from_pfn(frames[i], __PAGE_HYPERVISOR));
     }
 
-    xfree(pfns);
     return 0;
 
  fail:
@@ -4406,7 +4400,6 @@ long set_gdt(struct vcpu *v,
     {
         put_page_and_type(mfn_to_page(frames[i]));
     }
-    xfree(pfns);
     return -EINVAL;
 }