Re: [PATCH 1/3] x86/mm: do not attempt to convert _PAGE_GNTTAB to a boolean

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.05.2020 11:24, Roger Pau Monne wrote:
> 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)
>                         ^

This is a rather odd warning. Do they also warn for "if ( 0 )"
or "do { } while ( 0 )", as we use in various places? There's
no difference to me between a plain number and a constant
composed via an expression.

> Remove the conversion of _PAGE_GNTTAB to a boolean, since the and
> operation performed afterwards will already return false if the value
> of the macro is 0.

I'm sorry, but no. The expression was put there on purpose by
0932210ac095 ("x86: Address "Bitwise-and with zero
CONSTANT_EXPRESSION_RESULT" Coverity issues"), and the
description there is clearly telling us that this wants to stay
unless Coverity changed in the meantime. Otherwise I'm afraid
a more elaborate solution will be needed to please both. Or a
more simplistic one, like using "#if _PAGE_GNTTAB" around the
construct.

Jan