On Sun, Feb 17, 2019 at 07:34:20PM +1100, Michael Ellerman wrote:
> Balbir Singh <bsinghar...@gmail.com> writes:
> > On Sat, Feb 16, 2019 at 08:22:12AM -0600, Segher Boessenkool wrote:
> >> Hi all,
> >>
> >> On Sat, Feb 16, 2019 at 09:55:11PM +1100, Balbir Singh wrote:
> >> > On Thu, Feb 14, 2019 at 05:23:39PM +1100, Michael Ellerman wrote:
> >> > > In v4.20 we changed our pgd/pud_present() to check for _PAGE_PRESENT
> >> > > rather than just checking that the value is non-zero, e.g.:
> >> > >
> >> > > static inline int pgd_present(pgd_t pgd)
> >> > > {
> >> > > - return !pgd_none(pgd);
> >> > > + return (pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT));
> >> > > }
> >> > >
> >> > > Unfortunately this is broken on big endian, as the result of the
> >> > > bitwise && is truncated to int, which is always zero because
> >>
> >> (Bitwise "&" of course).
> >>
> >> > Not sure why that should happen, why is the result an int? What
> >> > causes the casting of pgd_t & be64 to be truncated to an int.
> >>
> >> Yes, it's not obvious as written... It's simply that the return type of
> >> pgd_present is int. So it is truncated _after_ the bitwise and.
> >>
> >
> > Thanks, I am surprised the compiler does not complain about the truncation
> > of bits. I wonder if we are missing -Wconversion
>
> Good luck with that :)
>
> What I should start doing is building with it enabled and then comparing
> the output before and after commits to make sure we're not introducing
> new cases.
>
Fair enough, my point was that the compiler can help out. I'll see what
-Wconversion finds on my local build :)
Balbir Singh.
