[PATCH v3 1/2] x86/mm: do not attempt to convert _PAGE_GNTTAB to a boolean

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Clang 10 complains with:

mm.c:1239:10: error: converting the result of '<<' to a boolean always 
evaluates to true
      [-Werror,-Wtautological-constant-compare]
    if ( _PAGE_GNTTAB && (l1e_get_flags(l1e) & _PAGE_GNTTAB) &&
         ^
xen/include/asm/x86_64/page.h:161:25: note: expanded from macro '_PAGE_GNTTAB'
#define _PAGE_GNTTAB (1U<<22)
                        ^

Remove the conversion of _PAGE_GNTTAB to a boolean and instead use a
preprocessor conditional to check if _PAGE_GNTTAB is defined.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Changes since v2:
 - Add comment.

Changes since v1:
 - Use a preprocessor conditional.
---
 xen/arch/x86/mm.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 53e3dcb2d4..e376fc7e8f 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1235,8 +1235,14 @@ void put_page_from_l1e(l1_pgentry_t l1e, struct domain 
*l1e_owner)
      *
      * (Note that the undestroyable active grants are not a security hole in
      * Xen. All active grants can safely be cleaned up when the domain dies.)
+     *
+     * NB: the preprocessor conditional is required in order to prevent clang's
+     * -Wtautological-constant-compare complaining about converting the result
+     * of a << into a bool is always true if it's evaluated directly in the if
+     * condition.
      */
-    if ( _PAGE_GNTTAB && (l1e_get_flags(l1e) & _PAGE_GNTTAB) &&
+#if _PAGE_GNTTAB
+    if ( (l1e_get_flags(l1e) & _PAGE_GNTTAB) &&
          !l1e_owner->is_shutting_down && !l1e_owner->is_dying )
     {
         gdprintk(XENLOG_WARNING,
@@ -1244,6 +1250,7 @@ void put_page_from_l1e(l1_pgentry_t l1e, struct domain 
*l1e_owner)
                  l1e_get_intpte(l1e));
         domain_crash(l1e_owner);
     }
+#endif
 
     /*
      * Remember we didn't take a type-count of foreign writable mappings
-- 
2.26.2