[PATCH][4.15] x86/shadow: replace bogus return path in shadow_get_page_from_l1e()

[Jan Beulich <jbeulich@xxxxxxxx>]

Prior to be640b1800bb ("x86: make get_page_from_l1e() return a proper
error code") a positive return value did indicate an error. Said commit
failed to adjust this return path, but luckily the only caller has
always been inside a shadow_mode_refcounts() conditional.

Subsequent changes caused 1 to end up at the default (error) label in
the caller's switch() again, but the returning of 1 (== _PAGE_PRESENT)
is still rather confusing here, and a latent risk.

Convert to an ASSERT() instead, just in case any new caller would
appear.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -802,9 +802,7 @@ shadow_get_page_from_l1e(shadow_l1e_t sl
     struct domain *owner;
 
     ASSERT(!sh_l1e_is_magic(sl1e));
-
-    if ( !shadow_mode_refcounts(d) )
-        return 1;
+    ASSERT(shadow_mode_refcounts(d));
 
     res = get_page_from_l1e(sl1e, d, d);